Background Sub-optimal medication adherence in pregnant women with chronic disease and pregnancy-related indications has the potential to adversely affect maternal and perinatal outcomes. Adherence to appropriate medications is advocated during and when planning pregnancy to reduce risk of adverse perinatal outcomes relating to chronic disease and pregnancy-related indications. We aimed to systematically identify effective interventions to promote medication adherence in women who are pregnant or planning to conceive and impact on perinatal, maternal disease-related and adherence outcomes. Methods Six bibliographic databases and two trial registries were searched from inception to 28th April 2022. We included quantitative studies evaluating medication adherence interventions in pregnant women and women planning pregnancy. Two reviewers selected studies and extracted data on study characteristics, outcomes, effectiveness, intervention description (TIDieR) and risk of bias (EPOC). Narrative synthesis was performed due to study population, intervention and outcome heterogeneity. Results Of 5614 citations, 13 were included. Five were RCTs, and eight non-randomised comparative studies. Participants had asthma (n = 2), HIV (n = 6), inflammatory bowel disease (IBD; n = 2), diabetes (n = 2) and risk of pre-eclampsia (n = 1). Interventions included education +/− counselling, financial incentives, text messaging, action plans, structured discussion and psychosocial support. One RCT found an effect  of the tested intervention on self-reported antiretroviral adherence but not objective adherence. Clinical outcomes were not evaluated. Seven non-randomised comparative studies found an association between the tested intervention and at least one outcome of interest: four found an association between receiving the intervention and both improved clinical or perinatal outcomes and adherence in women with IBD, gestational diabetes mellitus (GDM), and asthma. One study in women with IBD reported an association between receiving the intervention and maternal outcomes but not for self-reported adherence. Two studies measured only adherence outcomes and reported an association between receiving the intervention and self-reported and/or objective adherence in women with HIV and risk of pre-eclampsia. All studies had high or unclear risk of bias. Intervention reporting was adequate for replication in two studies according to the TIDieR checklist. Conclusions There is a need for high-quality RCTs reporting replicable interventions to evaluate medication adherence interventions in pregnant women and those planning pregnancy. These should assess both clinical and adherence outcomes.

Most commonly used medications such as paracetamol, most antibiotics and inhalers are considered safe for women to use during lactation. Most drugs taken by a breastfeeding woman will be expressed in small volumes in the breast milk. The amount depends on several factors, including the drug dose, the size of the molecule, the protein binding and lipid solubility of the drug, the age of the infant and volume of milk consumed. Data regarding short-term and long-term effects of maternal medication use on breastfed infants are limited. There is no direct evidence of impaired lactation with most commonly used medications, but some medications, such as decongestants (pseudoephedrine/phenylephrine), high-dose diuretics and the combined oral contraceptive pill, may inadvertently adversely affect maternal milk supply. Women need accurate and balanced advice regarding safety of medication in breastfeeding to avoid early or inappropriate cessation of medications in the lactation period. Learning objectivesUnderstand the pharmacokinetics of common medications used in the lactation period. Understand the impact of drugs on the breastfed infant. Be familiar with the current literature on drug safety and lactation to enable appropriate counselling.