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What is the greatest risk from a harmful exposure during very early pregnancy?

Harmful exposures during very early pregnancy have the greatest risk of causing miscarriage. A fertilized egg divides and attaches to the inside of the uterus during the first two weeks of embryo development. Very harmful exposures during this period (first four weeks after the first day of your last period) may interfere with the attachment of the embryo to the uterus. Harmful exposures during this time can also damage all or most of the cells of the growing embryo. Problems with uterine attachment and severe cell damage can both result in a miscarriage. Sometimes this miscarriage is before a woman even realizes that she is pregnant.

Less severe exposures during this time may only damage a few of the embryo’s cells. The cells of the embryo have a greater ability to recover at this early stage than they do later on in pregnancy. If a woman does not have a miscarriage, we believe that the exposures during this time are not likely to cause a birth defect.

We call the first four weeks of gestation the “all or none period.” “All” refers to high exposures damaging all of the embryo’s cells. This damage causes early miscarriage. “None” refers to exposures that are not high enough to have a significant effect on the pregnancy. We can use the rule of the “all or none period” to determine the risk of many different types of exposures. However, there are some important exceptions to this rule. Please contact MotherToBaby to discuss your specific exposure with our experts.