The first trimester is the highest-risk window of pregnancy for your developing baby. During these first 12 weeks, all major organs form, and the embryo is most vulnerable to substances, infections, and environmental exposures that can disrupt that process. Knowing what to steer clear of gives you real control over reducing those risks.
Alcohol Has No Safe Amount
No amount of alcohol is considered safe during pregnancy, and the first trimester is when the stakes are highest. The first two months are the critical period of organogenesis, when your baby’s brain, heart, spine, and facial structures take shape. Alcohol exposure during this window can cause physical malformations, particularly craniofacial abnormalities. Unlike some risks that are dose-dependent, even moderate drinking during these early weeks has been linked to lasting developmental effects. The safest approach is to stop drinking as soon as you’re trying to conceive or learn you’re pregnant.
Caffeine: Stay Under 200 mg Per Day
You don’t need to quit caffeine entirely, but you do need to cut back. The American College of Obstetricians and Gynecologists recommends no more than 200 mg per day, roughly equivalent to two standard cups of coffee. Keep in mind that caffeine also shows up in tea, soft drinks, chocolate, and energy drinks. If you’re combining sources throughout the day, it adds up faster than you might expect.
Pain Relievers That Raise Miscarriage Risk
Ibuprofen, naproxen, and other anti-inflammatory pain relievers (NSAIDs) are linked to a significantly higher risk of miscarriage. A large population-based study found that NSAID use during pregnancy was associated with an 80% increased risk of miscarriage overall. The timing matters enormously: when NSAIDs were taken around the time of conception, the risk jumped more than fivefold. Use lasting longer than a week carried an eightfold increase. Aspirin showed a similar pattern, though the data was less robust due to fewer users in the study.
The mechanism involves prostaglandins, hormone-like chemicals your body needs for successful implantation of the embryo. NSAIDs suppress prostaglandin production in reproductive tissues, which can interfere with that process. Acetaminophen (Tylenol) works differently, affecting prostaglandins only in the brain, and is generally considered the safer option for pain relief during pregnancy.
Prescription Medications to Flag
Several categories of prescription drugs can cause serious birth defects when taken during the first trimester. If you’re currently on any of these, talk to your prescriber before stopping or switching, since abruptly discontinuing some medications carries its own risks.
- Retinoids and high-dose vitamin A: Used for severe acne and some skin conditions, these are among the most well-established causes of birth defects. Excessive vitamin A from supplements or retinoid-containing drugs can harm fetal development.
- Certain seizure medications: Valproate poses the highest risk among anti-epileptic drugs. Others in this class, including carbamazepine, phenytoin, and topiramate, have been linked to craniofacial defects, growth problems, and oral clefts.
- Blood thinners (warfarin): Warfarin interferes with vitamin K and calcium binding during fetal development, leading to skeletal abnormalities and nasal underdevelopment.
- Certain antibiotics: Tetracyclines and fluoroquinolones should be avoided throughout pregnancy. The antibiotic streptomycin has been shown to cause irreversible bilateral deafness in babies exposed during the first trimester.
High-Mercury Fish
Fish is a valuable source of protein and omega-3 fatty acids during pregnancy, but certain species accumulate mercury at levels that can harm your baby’s developing nervous system. The FDA lists seven fish to avoid entirely: king mackerel, marlin, orange roughy, shark, swordfish, tilefish from the Gulf of Mexico, and bigeye tuna.
You should still eat fish. Aim for two to three servings per week (one serving is 4 ounces) from low-mercury options like salmon, shrimp, cod, tilapia, catfish, sardines, canned light tuna, pollock, and trout. These give you the nutritional benefits without the mercury exposure.
Foods That Carry Listeria Risk
Listeriosis is a bacterial infection that’s relatively rare in the general population but disproportionately dangerous during pregnancy, potentially causing miscarriage or stillbirth. The bacteria thrive in refrigerated, ready-to-eat foods. The FDA recommends avoiding:
- Deli meats, hot dogs, and luncheon meats unless reheated until steaming hot
- Soft cheeses made with unpasteurized milk, including queso fresco, queso blanco, and similar fresh cheeses (even pasteurized versions of queso fresco carry risk)
- Refrigerated smoked seafood labeled as lox, nova-style, kippered, or smoked, unless cooked into a dish like a casserole
- Refrigerated pâtés or meat spreads
- Unpasteurized milk and any products made from it
Cat Litter and Soil Exposure
Toxoplasmosis is a parasitic infection you can pick up from cat feces, contaminated soil, or undercooked meat. Most healthy adults barely notice the infection, but it can cause serious harm to a developing baby. The parasite becomes infectious one to five days after a cat sheds it, so daily litter box cleaning reduces the risk, but ideally someone else handles this task entirely during your pregnancy.
If you must clean the box yourself, wear disposable gloves and wash your hands thoroughly with soap and warm water afterward. The same precautions apply to gardening and handling sand from sandboxes, since outdoor cats often use garden beds and sandboxes as litter areas. Cover outdoor sandboxes when not in use, and always wear gloves when digging in soil.
Hot Tubs and Saunas
Raising your core body temperature above 38.9°C (about 102°F) during the first trimester has been associated with an increased risk of neural tube defects. Research on heat exposure found that in a hot tub set to 41°C (106°F), it took at least 10 minutes for a woman’s core temperature to reach that danger zone. In a cooler 39°C tub, it took at least 15 minutes. None of the study participants reached dangerous temperatures in a sauna before they felt too uncomfortable to stay.
The practical takeaway: brief, moderate heat exposure is lower risk than prolonged soaking. But because the first trimester is the most sensitive period for these types of defects, many providers recommend avoiding hot tubs and saunas altogether during early pregnancy, or limiting time to well under 10 minutes.
Exercise to Skip or Modify
Exercise during pregnancy is encouraged, but the first trimester is the time to drop activities with a high risk of falls, abdominal trauma, or collisions. International guidelines consistently flag these sports as ones to stop: contact sports like soccer, basketball, rugby, hockey, and combat sports; activities with fall risk like horseback riding, downhill skiing, water skiing, surfing, skating, gymnastics, and mountain climbing; and scuba diving, which poses unique pressure-related risks to the fetus.
Walking, swimming, stationary cycling, prenatal yoga, and moderate strength training are all safe alternatives for most pregnancies. If you were a regular runner before pregnancy, moderate running is generally fine to continue in the first trimester, but long-distance running and vigorous racquet sports land on the “avoid” list in most guidelines.
Household Chemicals Worth Reducing
Phthalates and BPA are synthetic chemicals found in plastics, food packaging, personal care products, and household items. Both can cross the placenta. Phthalates disrupt hormone signaling, acting as weak estrogens and strong anti-androgens, which can interfere with fetal development. BPA has been linked to metabolic disruption in both the mother and developing baby.
You can reduce exposure by avoiding microwaving food in plastic containers, choosing fragrance-free personal care products, opting for glass or stainless steel food storage, and checking labels for “BPA-free” on cans and bottles. These steps won’t eliminate exposure entirely, since these chemicals are widespread, but they meaningfully lower the dose your baby receives during this critical developmental window.