In this study, we found a high prevalence (82.6%) of xenobiotics (medicine, caffeine, nicotine and illicit drugs) in blood samples from first-trimester Danish pregnant women. To the best of our knowledge, this is the first study investigating the prevalence and pattern of xenobiotic substance use in early pregnancy in women unaware of the screening. This study was strengthened by the inclusion of samples from Danish pregnant women who generally had a very high attendance rate (95%) in prenatal screening programmes.24 Furthermore, this is the first study using this type of broad targeted substance screening on serum samples using UPLC-HR-TOFMS analysis, which can identify most relevant psychoactive medications, over-the-counter medications and illicit drugs found in the Danish population, although not the use of alcohol.23

Importantly, the cross-sectional study design only reflects the substance use within a short period of time, and it should be considered that habits may change during the pregnancy. Moreover, differences in the pharmacokinetics of various xenobiotics could also affect the likelihood of identifying a given substance. However, as the first trimester is the most sensitive and critical period in fetal development, it is critical to note that only 17.4% of pregnant women analysed in this study had no evidence of xenobiotic substances in their blood, indicating that early intervention is needed to prevent or regulate the use of medications during pregnancy. Due to ethical considerations, this analysis of samples from women unaware of the screening required anonymisation of the samples; hence, the present study is limited by the prevention of inclusion of perinatal outcomes in the analysis, which would very likely have been highly relevant in this context.

Pharmacological compounds available as over-the-counter drugs were the most frequent finding in this study. Acetaminophen was identified in a total of 9.4% of the samples. This finding raises concerns, as the use of acetaminophen during pregnancy might be associated with an increased risk of asthma,25 cryptorchidism,26 27 autism, hyperkinetic disorders and ADHD-like behaviour in children,28 29 despite not being associated with major birth defects.30 Thus, the markedly high prevalence of acetaminophen use in this and previous studies27–30emphasises the need for further research concerning the more subtle effects of acetaminophen in the human placenta and early development.

The prevalence of ibuprofen and other over-the counter drugs of the non-steroidal anti-inflammatory drug family (NSAIDs) associated with adverse outcomes, such as increased risk of miscarriage31 32 and congenital cardiac defects,33 was low (0.5%). Similarly, the frequency of antihistamine use was only 1.8%, which we consider surprisingly low as they are recommended for common conditions such as hyperemesis and allergies and have previously been reported to be widely used.34

In the present study, prescription drugs were found in 7.1% of the samples; other European studies have found a 79% prevalence of prescription drug use during pregnancy, with up to 48% of all pregnant women receiving a prescription in first trimester.7 35 This value likely reflects the design of this study, showing only a snapshot of the use of medicine among Danish pregnant women. The fact that certain types of drugs, such as antibiotics and antiemetic drugs, are not detected in the analytical method used28 also contributes to the differences between these and previously reported results. The analytical method used includes substances that are relevant in forensic cases.

The most frequent group of prescription drugs identified in our study was antidepressants, with a frequency of 3%. However, this was considerably lower than in a US study, which found that 6.6% of pregnant women used antidepressants.36 It remains unclear whether prenatal exposure to antidepressants and other psychoactive substances can impair brain development, cause postnatal neurobehavioural differences21 37 or increase the risk of miscarriage,38–40 major malformation,41–43 stillbirth and preterm birth.44

Among the prescription drugs identified, several are suspected of having adverse effects on fetal development and are thus not recommended for pregnant women (table 1). However, many of these drugs are also vital in the treatment of chronic diseases, highlighting the complexity of the safe use of medications in early pregnancy. Studies have shown that insufficient treatment of conditions such as depression,39 psychosis,45 epilepsy46 or inflammatory bowel47 disease during pregnancy can have negative effects on both the mother and child. Thus, when treating patients with psychiatric disorder or patients with other chronic diseases who are pregnant, it is important to consider the potential teratogenicity with the risk associated with the untreated diseases.39 45 47 However, a deeper understanding of lifestyle patterns and the intake of non-prescription medications is needed to limit the risks to necessary medical treatments, just as emphasis should be placed on avoiding unnecessary multidrug treatments. The use of medications among the pregnant population calls for continued optimisation of obstetric guidelines. Reflecting the use of such guidelines, metformin was only identified in 1.8% of our samples, which is in accordance with the Danish guidelines advocating discontinuation during pregnancy when used for treatment of diabetes or PCOS.

The presence of illicit drugs (0.9%) in our population appears relatively low compared with previous international studies, which have reported frequencies of up to 4.4%.12 18 A Danish study from 1998 found that 2.1% of Danish pregnant women had used cannabis and 0.2% had used other illicit drugs within the last 3 months before recognition of pregnancy.48 Naturally, some form of ‘healthy worker selection’ or change of abuse pattern prior to the doctor’s visit cannot be excluded, as cannabis is currently only approved for a severely limited number of conditions in Denmark and is presumably not used in the treatment of pregnant women. Thus, in other countries in which medical cannabis is legal, the prevalence rate among pregnant women may likely be higher.

Our findings show a high frequency of caffeine (76.4%), even though only 40% of Danish pregnant women reported drinking coffee in previous studies.49 50 This discrepancy may be explained by the intake of caffeine-containing beverages and energy drinks. Regardless of the source, caffeine may affect the fetus as it crosses the placenta51 and can cause miscarriage,52 53 fetal growth restriction54 55 and childhood leukaemia.56 Furthermore, animal studies indicate that caffeine exposure might potentiate the adverse effects of certain medications.57 58

We found evidence of (habitual) cigarette smoking in nearly 10% of the samples, despite the fact that smoking in pregnancy is a well-known risk factor59 associated with an increased risk of placenta dysfunction15 and adverse birth outcomes, such as low birth weight and preterm birth.14 60 As a previous report stated that 8% of pregnant women continue to smoke throughout pregnancy,61 this finding might reflect a continuous increase in smoking cessation during pregnancy. In recent years, prenatal exposure to cigarette smoke has also been associated with the development of diseases later in life, such as asthma and allergies,62 metabolic diseases63 and certain forms of cancer.14 Moreover, prenatal exposure to cigarette smoke has been associated with behavioural deficiencies and neuropsychiatric disorders.16 17 Many smokers reduce or quit smoking during pregnancy61; however, even short-term exposure during the first trimester may still result in adverse effects on the fetus. In this respect, first-trimester exposure to maternal cigarette smoke has been shown to reduce the number of gonadal cells in both male and female human fetuses.64 Thus, our findings underline the importance of maintaining focus on early smoking cessation during pregnancy.