Introduction: In the UK, around 40% of pregnant women are reported to use herbal medicinal products (HMPs) but the extent of herbal galactagogue (milk production stimulants) use during the postnatal period is unknown, longitudinal data on perceived effectiveness and safety is lacking and no studies have assessed the potential determinants of HMP use with a validated framework. Hence, a systematic review of the literature was carried out to identify safety issues related to HMPs to serve as the basis for a prospective questionnaire study. The primary aims of this prospective study were to collect data on HMPs at consecutive time-points throughout pregnancy and the postnatal period and report the prevalence of use of HMPs, including their perceived effectiveness and safety, any potential drug interactions (PDIs), potential behavioural determinants of use, and accuracy in reporting use in medical records. A secondary objective was to compare pregnancy outcomes to explore any potential differences between HMP users and non-users. Methods: The study was undertaken at Aberdeen Maternity Hospital (AMH) from March 2018 to June 2020. The initial self-administered questionnaire was designed to collect demographic information, information on HMP use (including perceived effectiveness and safety), other medications use (to assess potential drug interactions, PDIs), and potential behavioural determinants of use (assessed with Likert-scale items based on the Theoretical Domains Framework [TDF], a validated integrative framework used for behaviour change research). The follow-up telephone questionnaire was a simplified version of the initial questionnaire. Women were recruited during early pregnancy and contacted for follow-up at weeks 28 and 38 of pregnancy and one and six months after delivery. Women's medical records were accessed with consent to retrieve data on pregnancy outcomes and verify reporting of HMP use. All study data were managed with REDCap®, a username and passwordprotected electronic software, and analysed using SPSS statistics version 25. Results: The systematic review found HMP use during pregnancy may be associated with severe adverse effects, such as hepatotoxicity and increased risk of congenital malformations. A total of 252 women were recruited with a response rate of 34% and 24% (n = 60) of initial respondents completed the follow-up. 44% (n =112) of respondents reported taking an HMP at study completion and 9% (n = 10) of these women were taking herbal galactagogues while breastfeeding. Eight in ten women reported the HMPs they were taking were effective and 5% (n = 6) of HMP users reported adverse effects. Perceived knowledge, positive beliefs, family influence, and accessibility were found to be potential determinants of HMP use during pregnancy. A third of respondents were at potential risk of adverse effects from PDIs at study completion and the accuracy in reporting HMP use in electronic medical records was low (1%). There were no statistically significant differences in pregnancy outcomes between HMP users and non-users. Conclusions: Despite recognized safety issues on the literature and the widespread use of HMPs (alone or concomitantly with conventional medicines) reported in this study, HMP use is poorly recorded on women's medical records. Healthcare providers should actively engage with women about the use of HMPs to increase the accuracy in reporting use and educate patients on the potential benefits and harms associated with use during and after pregnancy.