Sensitive or Critical Periods of Development
In every pregnancy, a woman starts out with a 3-5% chance of having a baby with a birth defect.
This is called her background risk. This fact sheet talks about the critical periods of development
and types of birth defects that can result from exposures at different stages of pregnancy. This
information should not take the place of medical care and advice from your health care provider.
What are critical periods of development?
Each part of the baby’s body forms at a certain time in pregnancy. During this time, the body part
is very sensitive to damage caused by medications, alcohol or other harmful exposures. We call
this specific time the “critical period of development” for that body part.
Does the risk of certain types of birth defects change throughout pregnancy?
Yes, the risk depends on what body part is developing at the time of exposure. Once a body part
has formed, it is no longer at risk to develop major birth defects, but some exposures could still
affect its growth and function.
The chart on the next page shows the critical periods of development for different parts of the
body. The chart starts from the time of conception when the egg and sperm join. The weeks
listed on the chart are the “embryonic age” or “fetal age” of a pregnancy. Note that this is different
from a common way of dating a pregnancy called “gestational age.” Gestational age begins with
the first day of a woman’s last menstrual period. This day is usually two weeks before a baby is
conceived. This means that you can change gestational age to embryonic/fetal age by
subtracting two weeks. For example, 12 gestational weeks (since the day of your last period) is
the same as 10 fetal weeks (since the first day of conception).
The dark bars on the chart show when each part is most sensitive to harmful exposures and at
risk for major birth defects. Birth defects are typically classified as “major” if they cause significant
medical problems and need surgery or other treatment to repair. Heart defects, spina bifida, and
clubfeet are examples of major birth defects.
The light bars show periods when the parts are still at risk to develop minor birth defects and
functional defects. “Minor” birth defects by themselves do not cause significant medical problems
and usually do not require treatment or surgery. Minor birth defects can be variations of normal.
Wide-set eyes, large ears and certain birthmarks are examples of minor birth defects. Both major
and minor birth defects are physical, structural changes. However, “functional” defects change
how a part of the body works without changing its physical structure. Intellectual disability and
hearing loss are both examples of functional defects.
The chart also shows the location of the most common birth defects that can occur during each
week. In general, major defects of the body and internal organs are more likely to occur 3-12
weeks after conception. Minor and functional defects including those affecting the brain are more
likely to occur later in pregnancy.
