Table 3 Identified themes on barriers and strategies for recruitment, engagement, and retention

Ashman 2016 [55]

Clinical settings (Antenatal)

Pregnant people were recruited by Indigenous research assistants at one of two antenatal clinics (i.e., Indigenous antenatal birth services)

2010-present

Engagement & Recruitment:

• Time commitment is burdensome for people

Retention:

• Lack of continuity and irregularity of attendance at antenatal visit

Engagement & Recruitment:

• Community consultation – before the onset of the study, researchers engaged in a 2-year process of community consultation with Indigenous stakeholders, including Elders, mothers‚ schools, employment agencies and local Indigenous health organizations

• Hiring Indigenous staff to share expertise on community connections and knowledge

Retention:

• Building connection and trust between staff and participants

Bartholomew 2015a [37]

Clinical settings (Antenatal)

People were recruited in antenatal clinics with Lead maternity carer (LMC)

2009–2010

Engagement & Recruitment:

• Timeliness of engagement with partners involved (i.e., health care providers) and participants who are particularly in their first pregnancy, and people living in more socioeconomically deprived area

• Significant personal, health literacy-related barriers in disadvantaged group

Retention:

• Address the persistent equity issues in access to maternity care

Engagement & Recruitment: Not reported

Retention: Not reported

Bartholomew 2015b [56]

Clinical settings (Antenatal)

People were recruited in antenatal clinics with LMC

2009–2010

Engagement & Recruitment:

• Disparity in engaging marginalized and racialized people where people not engaging a maternity care provider were more likely to be non-European, < 20 years or > 40 years old, with poorer educational attainment, or living in more deprived households

Retention: Not reported

Engagement & Recruitment:

• Need for policies to improve engagement in antenatal care

Retention: Not reported

Bastain 2019 [57]

Community Health Clinics

People were recruited from four community health clinics

Nov 2015-Oct 2018

Engagement & Recruitment: NA

Retention: NA

Engagement & Recruitment:

• Established relationships with a lead clinic physician to engage and recruit participants

• Approach potentially eligible people at each clinic site and conduct in-person and telephone interviews

• Advertisements placed in local papers or in community locations indicating selected criteria for self-referral

• Retention:

• Participant tracking techniques to document changes in residential status

• Incentives such as small gifts timed to baby's milestones

• Consistent communication through quarterly newsletters physically and electronically mailed to participants

• Participant appreciation via semi-annual events to maintain contact

• Sustained strong clinical partnerships with community-based health care providers with high proportions of medically under-served and research underserved population

Begum 2012 [58]

Clinic settings (Physician offices)

Pregnant people were recruited in their first trimester in physician offices

Jun 2009-Jun 2010

Engagement & Recruitment:

• Barriers to engaging with other groups where participants are relatively homogenous and lack of diversity by ethnicity, maternal age, education, marital status, or family income

Retention: Not reported

Engagement & Recruitment: Not reported

Retention: Not reported

Chasan-Taber 2009 [59]

Clinical settings (Ambulatory obstetrical practices)

Prenatal care patients were recruited at their first prenatal care visit by bilingual recruiters

Jan 2006-Aug 2008

Engagement & Recruitment:

• Cancellation of appointments impact recruitment in clinic settings

• Inequities in engaging with people with sociodemographic representativeness in study population with participants being young and unmarried, consistent with prior studies Hispanic people encountering social inequities and barriers from very young childbearing ages

• Logistical challenges when recruiters miss potential participants and clinic staff that are already overburdened

Retention:

• Low attendance to pre-natal visits be due to personal and child sickness, domestic tasks, unanticipated employment opportunities, and partner restrictions

Engagement & Recruitment:

• Cooperation from clinic staff before the onset of the study with regular meetings to explain the purpose of the study and receive feedback on study protocols

• Bilingual recruiters in the clinic focused on recruiting patients at the time of their regularly scheduled prenatal care visit

• Recruiter training involving being female recruiters and bilingual to offer simple, clear information with flexibility and accommodation to people

• Pre-screening process to increase efficiency where recruiters used a limited number of available demographic (i.e., date of birth) and medical characteristics (i.e., date of last menstrual period) based on a daily roster of scheduled patients to generate a list of potential participants

• Culturally tailored materials such as eligibility screening forms, informed consent forms, HIPAA forms, questionnaires, posters, fliers, and patient handouts all translated into Spanish

• Participant compensation including variety of items (e.g., small teddy bear, baby t-shirt, hat and bib, gift certificates) with study logo and contact information

• Administer questionnaire in Spanish or English to eliminate potential language or literacy barriers

Retention:

• Flexibility in recruitment by retaining contact information to reach participants if the interview is interrupted

• Voucher (cafeteria) to complete the interview

• Monitoring recruitment goals through a data management system to track information about study recruitment

• Reduce participation burden by shortening length of questionnaire

• Shorten length of interview by collecting medical and obstetric history from the patient's medical record

Ernst 2015 [60]

Clinical settings and hospitals

People were recruited during the last trimester of pregnancy, during prenatal care visits at primary care by a gynecologist or in hospital by the attending obstetricians or project staff

Jan 2012-Mar 2013

Engagement & Recruitment:

• Time consuming to establish recruitment and other study procedures within the hospital settings

• Incentives only important to a limited extent e.g., those with low socio-economic status (SES)

• Study language limitations resulting in only involving people whose knowledge of the Ger- man language was sufficient to understand study materials and details of participation

• Variety of survey instruments and administration techniques to reach different populations

• Collaboration with multiple institutions, e.g., maternity units, primary care gynecologists, community services or other services for pregnant people

Retention:

• Need to examine further recruitment and (retention) strategies in the social context (e.g., community-based recruitment)

Engagement & Recruitment:

• Early communication about study and guideline for enrollment to inform people by primary care gynecologists, hospital staff (midwives, physicians) or by a study nurse

• Direct contact with expecting people & couples in the context of antenatal care (i.e., midwife consultations) by project members

• Engagement and recruitment by motivated gynecologist, nurse, recruiter increase likelihood of people taking part in a birth cohort

Retention: Not reported

Faro 2021 [61]

Clinic and community settings

People were recruited during pregnancy

NA

Engagement & Recruitment:

• Challenges in recruiting racially and ethnically diverse study participants

• Burden of biospecimen collection and data collection tools

Retention:

• High participant burden and stress of coordinating prenatal care combined with anxiety about birth event, changes in lifestyle and family expectations

• Social and family structure potentially a challenge for people to participate

Engagement & Recruitment:

• Recruitment in multiple languages to address potential literacy barriers

• Provide incentives for participation

• Record and keep track of reasons for non-enrollment

• Partnerships with organizations that work within the community of interest to recruit minority populations

Retention: Not reported

Garcia-Blanco 2018 [62]

Clinical settings

People were recruited during the third trimester of pregnancy

Jan 2015-Dec 2015

Engagement & Recruitment:

• Establish clinical-academic-community partnerships at all stages of the study

Retention: Not reported

Engagement & Recruitment:

• Clinical-academic-community partnerships can improve research efficiency and accelerate the recruitment and data collection phases of a study

• Incentives such as public library gift certificates and grocery gift certificates for completed questionnaires

• Convenience mailing with postage paid envelopes to minimize time and costs

Retention:

• Consistent communication such as phone calls for outstanding questionnaires

Gracie 2010 [63]

Clinical settings

People who receive prenatal viral serology testing were recruited through a partnership with clinical laboratory services

Sep 2009- Dec 2010

Engagement & Recruitment:

Not reported

Retention:

• Not reported

Engagement & Recruitment:

• Not reported

Retention:

• Consistent communication such as reminder phone calls for outstanding questionnaires

• Incentives, such as public library gift certificates and grocery gift certificates for completed questionnaires

Hertz-Picciotto 2010 [64]

Community settings

Telephone interviews, internet-based surveys, and home-based monitoring techniques were used to recruit families (one parent and one child) and older individuals (age 55 +)

Jul 2005-Jan 2007

Engagement & Recruitment:

• Participation motivation and passive refusal impacted by lack of time and interest

• Lack of trust impacted sampling from diverse communities, low SES, who are disproportionately impacted by toxins

Retention:

• Cohort length and time burden are important determinants of participation and retention

• Participant burden particularly for families with young children

Engagement & Recruitment:

• Oversampling of hard-to-reach populations can be effective in leading to a more diverse sample

Retention:

• Reduce participant burden time by conducting home visits

• Deploy the same study staff to build and maintain trust

Hertz-Picciotto 2018 [65]

Community settings (State-level agency that coordinates services for persons with developmental disabilities)

Families on the state-level agency list with a child with Autism Spectrum Disorder were mailed a letter notifying them about the MARBLES study, before or during a pregnancy, and once the woman became pregnant, they were invited to enroll

July 2005-Jan 2007

Engagement & Recruitment:

Not reported

Retention:

Not reported

Engagement & Recruitment:

Not reported

Retention:

Not reported

Kawamoto 2014 [66]

Clinical settings and Community (local government offices)

People were recruited at their first prenatal examination at cooperating health care providers (obstetric facilities) and through community-based recruitment at local government offices

Jan 2011-Mar 2014

Engagement & Recruitment:

Not reported

Retention:

Not reported

Engagement & Recruitment:

• Consistent communication with participants via telephone calls to continue engagement

Retention:

Not reported

Lara-Cinisomo 2016 [67]

Clinical settings

People were recruited during prenatal visit and community centres

Jul 2013-Apr 2014

Engagement & Recruitment:

• Issues of confidentiality can be especially important to vulnerable populations, such as pregnant and Spanish-speaking people

• Study setting (and traveling to study site) an important indicator of Latina people likely to participate and enrol in the study

• Conduct community-based needs assessments to determine research areas of interest, mental health needs among perinatal native and U.S.-born Latin people to engage and recruit

• Fear of being rude to refuse participation

Retention:

• Geography as a barrier when many live outside of the study catchment area and must travel long distance to receive prenatal care

• Culturally appropriate methods to ensure participation and retention

Engagement & Recruitment:

• Bilingual investigators and recruiting staff to enrol participants

• Consistent communication by providing a cell phone number for participants to inquire about the study

• Availability and accessibility of PI when greeting people at in-person and phone interviews to build trust

• Establish trust early in the study (at enrollment) by providing participants brief reports and preliminary results from the study

• Build trust and relationships by assuring participants that information will not be shared with government agencies

Retention:

• Consistent communication such as follow-up and confirmations letters at all points to thank participants and provide reminders of future appointments

• Translation of material in participants’ preferred languages (English and Spanish)

Loubet 2016 [68]

Online

Pregnant people were enrolled through a web-based platform

Nov 2014-Apr 2015

Engagement & Recruitment:

• Non-representativeness of study sample

• Challenges with "volunteer effect"—those who choose to volunteer for studies may differ in lifestyle and health from those who decline)

Retention:

• Longer follow-up periods have higher chances for dropout

Engagement & Recruitment:

• Web-based survey cover a wider geographical distribution of participants and achieve a high active participation rate

Retention: Not reported

Loxton 2015 [38]

Online

People were recruited for the longitudinal online survey. Promotions were made via social media (Facebook), promotion, by referral, web activities, and via traditional media outlets

Oct 2012-Dec 2013

Engagement & Recruitment:

• Referrals are most likely and social media less likely to support engagement and recruitment

• Costs of advertising will need to be considered

• Challenges in recruiting hard-to-reach populations via social media channels

Retention: Not reported

Engagement & Recruitment:

• Social media most successful to recruit young people and achieve demographic diversity

Retention: Not reported

McDonald 2013 [86]

Clinical and Community settings

People were recruited from health care offices, communities, and through Calgary Laboratory Services

May 2008-Dec 2010

Engagement & Recruitment:

• Non-response bias throughout the study need to be understood through social contexts, inequities and wellbeing

Retention: Not reported

Engagement & Recruitment:

• Incentives such as library and grocery store gift cards. In order to keep participants engaged and updated

• Milestone communication such as congratulation cards baby’s birthdays

• Study newsletters and updates on preliminary results

Retention: Not reported

Manca 2013 [69]

Clinical settings (Physician offices)

People were recruited face-to-face in physician offices, distributing posters and pamphlets, word-of-mouth, media, and the Internet

May 2009-Nov 2010

Engagement & Recruitment:

• Geography and study setting difficult to reach site by transit

• High SES participants more likely to participate in the APrON

Retention: Not reported

Engagement & Recruitment:

• Variety of recruitment strategies: 1) Face-to-face recruitment through physician offices and high-volume maternity clinics; 2) Honorarium for clinic staff to discuss the study with patients and attain contact information; 3) Advertising (posters and flyers in public spaces that pregnant people were likely to visit; 4) Word-of-mouth from health care providers, staff, friends and family of participants; 5) Media outlets with the greatest readership and audiences, as well as those related to pregnancy and nutrition; 6) Internet

• Face-to-face strategies in physician offices were the most successful in recruiting people

• Collaboration with relevant organizations (Doula Association, Association for Safe Alternatives in Childbirth) and professionals (midwives and naturopath clinics), centres for pregnant teens, programs to support low-income pregnant people, community perinatal programs and the provincial after hours medical help line enhanced recruitment strategies

Retention: Not reported

Morrens 2017 [81]

Clinical settings (Maternity hospitals as primary sampling units (PSU))

Pregnant people were informed midwives and invited to participate. Study nurses provided potential participants with detailed information about the study protocol and recruited participants

Nov 2013-Nov 2014

Engagement & Recruitment:

• Overcoming an intuitive ‘no’ by eliminating fear or perceived danger of being asked to participate

• Mistrust by provided information about data use

• Identify perceived benefit by reassuring participants that study participation would not involve risks

• Frame the utility and benefits of study participation more in terms of ‘personal profit’ by offering more personalized information on how to avoid or protect themselves against exposure to environmentally hazardous chemicals

• Avoiding research triage as socially vulnerable people often leave the hospital after delivery and are more difficult to reach for the purpose of research studies

Retention: Not reported

Engagement & Recruitment:

• Community engagement and consultation to modify study procedures and ask questions (how do socially vulnerable pregnant people experience environmental health risks); and understand participation barriers for human biomonitoring research (which study procedures may cause barriers for socially vulnerable people?), identify opportunities to increase participation (how can we motivate pregnant people?)

• Network with community organizations and local professionals to achieve broader publicity and endorsement of the study

• Stimulate word-of-mouth promotion within communities in the catchment areas

• Implement a personal buddy system for participants to build trust and personal relationships with potential participants

• Invest in information transfer to eligible candidates about the study process in the pre-parturition period

Retention: Not reported

Morton 2014 [70]

Clinical settings

Lead maternal carers recruited pregnant people into the study at routine appointments

Apr-25 2009-Mar 25, 2010

Engagement & Recruitment:

• Need to address challenge of recruiting people from diverse communities an ongoing issue

Retention:

• High attrition due to missing contact information for follow-up appointments

Engagement & Recruitment:

• Consultation and collaboration strategies with relevant health care and community organizations

• Communicating and working with diverse population to enable antenatal enrollment

• Promote awareness through indirect and direct contact with prospective participants and communities

• Advertising and media locations in areas and languages most frequently spoken in the study region

Retention:

• Up to date contact details to reduce attrition

• Establish good rapport between interviewers and participants to build trust

• Translation and relationship building to match interviewers with participants based on ethnicity, language, and availability (preferred time and location)

• Flexibility of scheduling to minimize inconvenience and burden for participants

• Satisfaction with the study and continued engagement

• Address attrition by reducing dissatisfaction with the study or its methods

• Auditing is essential to manage in-person antenatal interviews by an external research company for assuring accurate data acquisition

• Continuing engagement strategies and regular contact with the participants to provide access to resources, initial findings, information regarding privacy and confidentiality, and electronic copies of the participant newsletters

Postma 2016 [71]

Community settings

Subjects were recruited via household-based door-to-door within randomly selected, pre-determined geographical areas

2010–2011

Engagement & Recruitment:

• Need variety of approaches to promoting outreach and engagement strategies across different media outlets (i.e., English and Spanish)

• Lack of clarity on which certain strategies led more credibility than other strategies

Retention: Not reported

Engagement & Recruitment:

• Application of cultural responsiveness theory helpful to structure outreach and engagement, including:

• assembling a culturally competent team;

• partnering with community organizations;

• creating a personalized marketing and media campaign

• Hiring locally so the staff hired reflected the county, and represented a cross-section of age, gender, ethnicity, and academic backgrounds

• Staff serving as “bridge” between researchers, community partners and community members at-large

Retention: Not reported

Quante 2012 [72]

Clinic and community settings

Subjects were recruited at university hospitals, local clinics, public health centres, kindergartens, schools and partner study centres

Jul 2011-Jul 2014

Engagement & Recruitment: Not reported

Retention: Not reported

Engagement & Recruitment:

• Consistent communication with participants throughout the stages of the study

• Study promotion through presentations, forums, fliers/posters, press releases and TV ads

Retention:

• Build relationships with participants involving invitations and opportunities to participate in parties and sport events for children

Richiardi 2007 [73]

Online (Internet-based)

Registered people were asked to complete three Internet-based questionnaires during pregnancy and 6 and 18 months after delivery and consent for passive follow-up of the child and the mother, using linkage with records of health-related databases, such as the Hospital Discharge Registry and cancer registries

July 2005-Dec 2006

Engagement & Recruitment:

• Selection bias with participants being different from the general population of pregnant people both due to access to the Internet and were self-selected volunteers (highly educated, high SES and older)

• Internet-based recruitment poses concerns about privacy

Retention:

• Lack of contact with the participants, creating challenges for collection of biological samples

Engagement & Recruitment:

• Multiple promotion strategies such as posters at the main hospitals of the city; leaflets enclosed with the results of laboratory tests and ultrasounds carried out for the prenatal screening; leaflets distributed at the pre-delivery classes after giving a brief overview of the study

• Strategies for Internet-based recruitment is a more long-term and sustainable approach over several years including period of recruitment and follow-up

• “Anonymity” of an Internet-based questionnaire may yield higher response rates, and more accurate responses, for questions of a personal nature

Retention:

• Internet-based recruitment and engagement facilitates communication with participants and reduces loss to follow up

Smith 2021 [74]

Community settings

People were recruited via traditional, social media and broadcast email outreach

Apr 2018-Jan 2020

Engagement & Recruitment:

• Social media as an approach alone may not be entirely effective as recruiting hard-to-reach people who are less likely to enroll and participate in the study

• Lack of interest or distrust in scientific research

• Disapproval from family and friend

• Time/participant burden

Retention: Not reported

Engagement & Recruitment:

• Social media has the potential to reach hard to recruit populations such as people experiencing pregnancy-related depression, but in combination with other traditional strategies and efforts to build trust, relationship, and acknowledge institutional royalty may be more effective

• Perceived personal benefit may motivate people to participate (i.e., improved pregnancy outcome, health education, and improvements to their own health

• Social media recruitment requires minimal efforts and cuts time burden

Retention: Not reported

Spallek 2020 [75]

Clinical settings

People were recruited by gynecologists and midwifes as well as via ads and flyers

2016–2018

Engagement & Recruitment:

• Lower response rates impacted by participants being from migrant backgrounds and lower educated families

Retention: Not reported

Engagement & Recruitment:

• Culturally sensitive recruitment and bilingual study materials and study nurses to recruit and follow-up as many pregnant people as possible independent from their social or cultural background

Retention: Not reported

von Ruesten 2014 [76]

Clinical settings

Subjects were sent mail invitations after a routine ultrasound examination at local hospital

1999–2008

Engagement & Recruitment:

• Ensuring representativeness of all pregnant people and fathers in the region

Retention: Not reported

Engagement & Recruitment:

• Not reported

Retention: Not reported

Walker 2011 [77]

Clinical settings (Antenatal)

Recruitment was performed by experienced obstetrical research personnel

Oct 2002-Apr 2009

Engagement & Recruitment: Not reported

Retention: Not reported

Engagement & Recruitment: Not reported

Retention: Not reported

Webster 2012 [78]

Clinical settings (Hospital)

People were recruited by clinic staff and various promotions strategies (i.e., advertising mediums, website, media, online and recruitment emails, as well as a study booth at baby “trade shows”)

Oct 2006-Feb 2008

Engagement & Recruitment:

• Direct contact via physicians and midwives—expensive and unsuccessful, due to late start in implementing this strategy, the difficulty of communicating directly with clinicians (e.g., by phone or email), the high volume of research requests, concern about the study topic, and the low numbers of eligible people visiting a clinic on any given day

• Challenge with recruiting people from diverse groups, younger, ethnically diverse (predominantly Caucasian), less affluent and educated

Retention: Not reported

Engagement & Recruitment:

• Involving prominent members of the clinical community (senior midwives, family physicians and obstetricians) from the beginning of the study to facilitate recruitment

• Word-of-mouth as the most cost-effective recruitment method (i.e., forwarding of messaging to friends and family, referral from another study, and direct recruitment emails)

• Availability of posters and flyers where people frequently visit (participating hospitals, at family practice clinics, midwifery clinics, medical testing laboratories, an ultrasound clinic, maternity and newborn retail outlets, libraries, community centres, yoga studios, grocery and natural foods stores, coffee shops, community message boards, and at select naturopathic, chiropractic, massage therapy, and physiotherapy clinics)

• Online recruitment methods effective when there is comprehensive study information

• Recruitment booths (CHirP study recruitment booth) at baby and pregnancy trade shows, family physician, midwifery and doula conferences, an outdoor summer yoga event and at summer farmers’ markets

• Raise awareness and knowledge sharing/exchange about recruitment strategies with care providers via powerpoint presentations to obstetricians, family physicians, midwives, nurses and other hospital staff during research rounds and staff meetings at the three participating hospitals

• Hiring enthusiastic and knowledgeable study staff to build relationships and trust

• Incentives such as hospital parking passes or bus tickets, a baby T-shirt, and their personal results at the end of the study (chemical levels measured in their blood and homes, shared at least 1 year after all babies had been born), offering genuine thanks via small gifts and cards, and offering to share personal results with participants helped us to recruit 152 people ≤ 15 weeks gestation within 17 months; and follow up visits available when needed

Retention: Not reported

Zook 2010 [79]

Clinical settings

People were recruited by clinic staff at four sites (Baltimore, Boston, New York, and St. Louis)

Feb 2005-Mar 2007

Engagement & Recruitment: Retention: Not reported

Retention: Retention: Not reported

Engagement & Recruitment:

• Keeping contact to help locate participants who move or change phone numbers by asking mothers at the initial study visit, to provide the names, telephone numbers, and addresses of up to three family members, neighbors, or friends who would be willing to help the staff communicate with the mother

• Establish and build relationships with clinic personnel at recruitment sites to increase study awareness

• Hire culturally competent and culturally sensitive staff with strong interpersonal skills

• Identify supporting clinics in the catchment area

• Promotions and presentations at child health community programs and in clinic areas

Retention:

• Monitor staff assignments and effectiveness of recruiting strategies with assignment delegation

• Establish minimum number of required call attempts for completion of study calls/visits

• Collect name/contact information for alternate contacts at the initial interview

• Allow for after-hours staffing to complete calls

• Purchase site cell phones to use for calling participants who are difficult to reach and who may not answer a hospital number

• Tracking/monitoring reports to chart the study progress and identify problem areas

• Mail appointment letters and follow-up letters to participants who are difficult to reach, monthly postcards as reminders of study events or check-in requests, and greeting cards