It doesn’t appear that there is any conclusive evidence that taking the diphenhydramine when pregnant is detrimental. Then again, no one wants to experiment on a pregnant human so the best data we have comes from rabbits and rats. In any case, to be safe, always consult your obstetrician before taking Benadryl or Sominex or any other over the counter drug containing diphenhydramine.

The compound has different purposes. Sometimes found in antihistamines to treat allergies and also in sleep aids to treat insomnia.

According to the US Food and Drug Administration (FDA) diphenhydramine is a Category B drug. The FDA uses a pregnancy category system to rate risks to a fetus when specific medicines are used. Category B is awarded to those medicines that haven’t been tested fully in pregnant humans but do not seem to cause harm in controlled animal studies.

In past animal studies, large doses of the drug were given to pregnant rabbits and/or rats. Fetal development was unimpaired and there was no increase in risks of any kind. There have been human studies, but none that have produced enough data to determine safety of the drug for pregnant women. So, diphenhydramine seems to be fairly safe for pregnant women but that’s not conclusive.

There is some evidence that suggests diphenhydramine taken during the final two weeks of pregnancy may increase the retinopathy of prematurity (an eye problem in premature babies that leads to blindness).

Some women use diphenhydramine “off label” as a treatment for morning sickness. Again, always talk to your doctor before taking any over the counter drug for any purpose. Doctors will occasionally recommend diphenhydramine for extremely severe cases of morning sickness. It is not recommended for light or even average symptoms.

Source: http://www.naturalfamilyonline.com/basics/health-and-wellness/diphenhydramine-and-pregnancy