According to Drugs.com, the FDA assigns a pregnancy class C to two of the three active ingredients in DayQuil, placing it on the list of drugs not recommended during pregnancy. The third ingredient, acetaminophen, has no formal pregnancy category, and doctors believe it is safe for fever and short-term pain relief during all stages of pregnancy.
The manufacturer, Vicks, says "If pregnant or breast-feeding, ask a health professional before use." WebMD recommends taking DayQuil only when necessary due to "evidence for malformations." There is also evidence this combination of drugs has the potential to "reduce uterine blood flow causing fetal hypoxia."
While the FDA does not assign a pregnancy class to DayQuil itself, Drugs.com reports on the individual ingredients. Tests of dextromethorphan on chicken embryos show it causes abnormalities in development; however, there are no reports of controlled data from human pregnancy. Phenylephrine, the other category C ingredient in this preparation, receives its warning without controlled studies in either animals or human pregnancy. Doctors recommend DayQuil only when there is no other option and the benefits outweigh the risks.
Pregnant women who experience nasal stuffiness do have safe alternatives. Saline only nasal sprays help to reduce mucus production. In addition, BabyMed says, "Loratadine, or Claritin, is an antihistamine, and is usually considered safe to take during pregnancy."