A search for “FDA and WHO pregnancy safety guidelines by trimester” suggests that there might be a single checklist of safe and unsafe choices for each stage of pregnancy. There is no such joint document. The US Food and Drug Administration (FDA) provides information on medicines and food safety, while the World Health Organization (WHO) issues recommendations on antenatal care and public health. Their guidance can be read together, but it must be applied to the individual pregnancy, the local health system, and the specific decision at hand.
Timing matters. An exposure during early fetal development may raise a different question from the same exposure later in pregnancy. Conversely, some precautions apply throughout all three trimesters. Recent research also shows why “safe in pregnancy” cannot always be reduced to a yes-or-no label: the health effects of an untreated condition must be considered alongside the potential effects of its treatment.
First trimester: establish care and review exposures
Begin antenatal care early
The WHO recommends a first antenatal contact within the first 12 weeks of pregnancy. Its model schedules at least eight contacts overall, providing opportunities to assess health, identify complications, and adapt care as pregnancy progresses. These are contacts with a health professional, not eight identical examinations; additional care may be needed for a particular condition. World Health Organization (WHO)
Early care establishes gestational age and provides a chance to review existing illnesses, prescribed medicines, over-the-counter products, and supplements. The WHO recommends daily supplementation with 30–60 mg of elemental iron and 400 µg of folic acid during pregnancy, although a clinician may adjust an individual regimen. Folic acid is particularly relevant before conception and in early pregnancy, so a person who is planning pregnancy should discuss it before a positive test where possible. who.int
Assess medicines by their evidence, indication, and timing
The FDA no longer assigns prescription medicines the old pregnancy letter categories A, B, C, D, and X. Its Pregnancy and Lactation Labeling Rule instead calls for a risk summary, clinical considerations, and supporting data. This format encourages a more useful question: what is known about this medicine for this condition, at this dose and stage of pregnancy, and what are the risks of leaving the condition untreated? FDA
A medicine review should therefore happen promptly, but an established treatment should not be stopped solely because pregnancy has been confirmed. The relevant comparison is often between treatment options and the underlying disease, rather than between treatment and a hypothetical pregnancy without illness. If human pregnancy data are limited, that uncertainty should be stated plainly. FDA pregnancy exposure registries are one way researchers collect additional information after medicines reach clinical use. FDA
Second trimester: use gestational age precisely
Understand the 20-week NSAID warning
“Second trimester” is too broad a category for some medication decisions. The FDA recommends avoiding nonsteroidal anti-inflammatory drugs (NSAIDs) at approximately 20 weeks of pregnancy or later, unless a clinician specifically advises their use. NSAIDs include ibuprofen and naproxen. Exposure from that point can, rarely, impair fetal kidney function and reduce amniotic fluid. At around 30 weeks and later, there is an additional concern about premature closure of the fetal ductus arteriosus. The FDA notes an exception for low-dose 81 mg aspirin prescribed for certain pregnancy-related conditions. fda.gov
This is an example of why FDA and WHO pregnancy safety guidelines by trimester should be treated as a framework for questions, not a substitute for checking the exact gestational week. A person prescribed low-dose aspirin should not confuse that supervised treatment with taking an over-the-counter NSAID for pain.
Use screening and imaging to guide care
The WHO recommends one ultrasound examination before 24 weeks as part of routine antenatal care. Its purposes include estimating gestational age and improving the detection of multiple pregnancies and fetal anomalies. An ultrasound recommendation does not imply that every possible condition can be identified or excluded by a scan. who.int
The second trimester is also a period in which findings may change a care plan. WHO guidance issued in 2025 specifically addresses diabetes during pregnancy, including education, glucose monitoring, treatment, and further assessments. The existence of these detailed recommendations illustrates an important point: a general pregnancy safety article cannot supply one appropriate diet, medicine, or monitoring schedule for everyone with gestational or pre-existing diabetes. who.int
Third trimester: continue surveillance and prepare for birth
Keep antenatal contacts responsive to changing risks
In the WHO eight-contact model, visits become more frequent toward the end of pregnancy, with scheduled contacts at 30, 34, 36, 38, and 40 weeks. Their value lies in reviewing maternal and fetal health and responding to problems as they arise. Someone with symptoms or a known complication should seek assessment when needed rather than wait for the next scheduled contact. who.int
Recent research helps explain why treating maternal illness can be part of pregnancy safety. In the randomized CHAP trial, treating mild chronic hypertension during pregnancy reduced a composite of adverse pregnancy outcomes compared with waiting until hypertension became severe; it did not increase the trial’s measured incidence of infants who were small for gestational age. The finding concerns a defined group and a supervised treatment strategy. It does not establish a universal medicine or blood-pressure plan for every pregnancy. nejm.org
Recheck medicines and preventive care
As delivery approaches, clinicians may revisit the indication, dose, and timing of medicines, including those started earlier in pregnancy. The NSAID warning becomes especially relevant after about 30 weeks because of the fetal cardiovascular concern described by the FDA. Decisions about preventive treatment also depend on a person’s risk factors and local recommendations. For example, WHO identifies low-dose aspirin for people at high risk of pre-eclampsia and calcium supplementation in populations with low dietary calcium intake as measures to consider within appropriate care. Neither should be started from a general article alone. fda.gov
The practical purpose of FDA and WHO pregnancy safety guidelines by trimester is to prompt a fresh review as circumstances change. A plan made at eight weeks may need revision after a new diagnosis, a scan result, or a change in symptoms.
Precautions and evidence questions that span all trimesters
Apply food safety guidance throughout pregnancy
Food safety is not limited to early pregnancy. The FDA highlights Listeria monocytogenes as a particular concern and advises care with foods such as unpasteurized milk products and certain refrigerated, ready-to-eat foods. Its pregnancy resources also explain safe selection, handling, and preparation practices. FDA
Fish illustrates why useful guidance is more precise than “avoid seafood.” FDA and US Environmental Protection Agency advice encourages people who are pregnant to choose a variety of lower-mercury fish: generally two to three adult servings per week from its “Best Choices” list, or one from its “Good Choices” list. The appropriate choice also depends on food preparation and any local advisory for fish caught by family or friends. U.S. Food and Drug Administration
Read new studies alongside their methods
Acetaminophen, also called paracetamol, demonstrates how evidence and agency communications can differ. In 2025, the FDA initiated a process to change its labeling to reflect evidence of a possible association between use during pregnancy and certain neurodevelopmental conditions. The WHO stated that there was no conclusive scientific evidence confirming a link with autism. These statements do not establish that the medicine causes autism. fda.gov
A large 2024 Swedish cohort study found that associations seen in conventional analyses were absent in its sibling-control analysis. A 2026 systematic review and meta-analysis likewise found no evidence of increased autism, ADHD, or intellectual-disability risk in the evidence it assessed. Observational research still has limits, including uncertainty about precisely when and why a medicine was taken. For an individual facing pain or fever, the sound approach is to discuss the symptom and suitable treatment with a healthcare professional, rather than infer a personal treatment rule from a headline. jamanetwork.com
Conclusion
FDA and WHO pregnancy safety guidelines by trimester are most useful when read as complementary sources. WHO antenatal recommendations provide a structure for continuing assessment; FDA labeling and safety communications help evaluate particular medicines and foods. Gestational week, medical history, dose, reason for treatment, and the strength of the evidence all affect the decision.
Current science does not provide a universal safety verdict for every exposure in every pregnancy. Early antenatal care, careful review of medicines, attention to food safety, and reassessment when health circumstances change provide a more reliable guide across all three trimesters.
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