The first 10 weeks of pregnancy (8 weeks of fetal development) are called the embryonic period; it is a time of extremely important development in the baby. The embryo is most susceptible during this time to factors that can interfere with its development. Most birth defects occur during this period.
Birth Defects
When a birth defect occurs, we want to know why it happened. This can be frustrating because in most instances we cannot determine a cause. Teratology is the study of abnormal fetal development. A substance that causes birth defects is called a teratogen or is said to be teratogenic. Some things may have a bad effect (be teratogenic) at one point in pregnancy, then be safe at others.
The most critical time appears to be early in pregnancy, during the first trimester or first 13 weeks. An example of this is rubella (German measles). If the fetus is infected during the first trimester, abnormalities such as heart defects can occur. If infection happens later, problems are often less serious.
Effect of Medications on the Baby
Medications can be grouped into three main groups—safe, unsafe and unsure. It's best to avoid any medication during pregnancy unless you discuss it with your doctor. Some medications, such as thyroid medication, are necessary and important during pregnancy.
It's easier and safer to discuss medication use with your physician ahead of time rather than after you have taken a medicine and want to know if it is safe or if it could harm your baby. For a chart on how some medications can affect the developing fetus.
Other medications, besides those listed on the chart, may also be harmful. If you take any of those listed below during your pregnancy, don't panic! Exposure alone doesn't mean definite harm to the fetus. The effect on a developing fetus depends on when you took the medication, the amount you took and how long you took it. Talk to your doctor if you believe you took any of the medications listed below:
 | angiotensin-converting enzyme (ACE) inhibitors |
| aminopterin |
| anticonvulsants |
| benzodiazepines |
| ethanol |
| etretinate |
| live vaccines |
| methimazole |
| penicillamine (not penicillin) |
| ribavirin |
| vitamin A (in large doses) |
Cataracts
Congenital (present-at-birth) cataracts rarely happen and are usually genetic. With cataracts, the lens of the eye is not transparent. Children born to mothers who had German measles (rubella) around the 6th or 7th week of pregnancy may be born with cataracts.
Air Pollution
I am often asked if smog an expectant mother breathes will harm her unborn baby. This is rarely a problem and would be very hard to prove. Your lungs and airways filter the air you breathe, and that protects your baby. However, exposure to cigarette smoke—from the mother's smoking or from the mother's exposure to secondary smoke—is harmful to a developing baby.
We believe exposure to cigarette smoke may be harmful to an unborn baby.
