INTRODUCTION
Non-adherence to pharmacological treatment is a significant global
public health issue, as it is estimated that half of patients do not
take their medication as prescribed [1, 2]. Medication adherence is
defined as the extent to which medication-intake behavior corresponds
with the medication regimen agreed upon with the healthcare professional
[1]. Among pregnant women, non-adherence can result in or facilitate
the onset of maternal and fetal complications, enhance risks of
hospitalization, and subsequently increase healthcare costs [2, 3].
Women tend to overestimate the teratogenic risk of medication [4-6],
resulting in reluctance to use medication during pregnancy due to these
concerns [6-9]. Incorrect perception of the safety of medication use
during pregnancy is widespread [10], and leads to caution among
treating physicians in prescribing medication as well [11-13].
Previous studies generally indicate low adherence to prescribed
medication during pregnancy, but methods and definitions differed
substantially between studies. When comparing dispensing data to
self-reported medication use, only 43% of all medications dispensed
were reported to be used by Danish pregnant women, with notable
differences between medications for chronic conditions (70-100%) and
medications used for local or short-term treatment (12-77%) [14].
Similarly, in a Dutch cohort study among pregnant women concordance
between self-reported medication use and pharmacy records was moderate
to good for medications used for chronic conditions, whereas concordance
was low for medications that are used occasionally [15]. Instead of
using a self-administered questionnaire to verify actual use of
prescribed medication, De Jonge et al. conducted telephone interviews,
showing that 84-92% of mothers confirmed taking the medication, but
slightly more than half of the medications were used at the prescribed
dose and for the duration prescribed [16].
Establishing the extent to which pregnant women are adherent to their
medication is challenging. Methods that rely on self-reported medication
use, including questionnaires and interviews, are prone to
underreporting [17-22], resulting in unfairly low estimates of
adherence when being compared to prescription databases. More reliable
methods, such as electronic drug monitoring, pill counts, and detection
of medication in biological samples, are often not feasible in
large-scale studies because of high costs and logistical challenges.
Nevertheless, more insight into adherence to pharmacological treatment
among pregnant women is needed not only to improve pharmacotherapy
outcomes, but also to guide studies on medication safety during
pregnancy, which increasingly rely on administrative healthcare
databases and registries [23]. Therefore, we aimed to determine to
what extent women adhere to treatment recommendations regarding
prescribed medication during pregnancy using diaries as the reference
standard, which are less prone to underreporting compared to methods
that rely on recall over a longer period of time [24, 25].