A typical pregnancy lasts about 40 weeks (280 days). Each part of a baby’s body forms at a specific time during pregnancy, during which the body can be sensitive to damage caused by harmful exposures, such as medications, smoking, and alcohol.
There is no one specific week that is most critical in pregnancy, but rather, periods of development and types of birth defects that can result from exposures at different stages of pregnancy.
What Can I Expect in the First Trimester of Pregnancy?
The first trimester:
- The first trimester is the earliest stage of pregnancy that starts on the first day of a woman’s last menstrual period up to the 14th week of pregnancy (13 weeks and 6 days)
- This is a critical time for development for the baby
- In this early stage, the baby’s body structure including the spine, head, arms, and legs, and organ systems including the heart, stomach, and lungs develop
- Most miscarriages occur during the first trimester, before the 12th week of pregnancy
- Major birth defects are most likely to occur from harmful exposures during this time
What Can I Expect in the Second and Third Trimesters of Pregnancy?
The second and third trimesters:
- Growth of structures and organs that developed during the first trimester occurs during these trimesters
- The risk of miscarriage is reduced, but infections or abnormalities of the uterus or placenta can cause a miscarriage in the second trimester
- Placental abruption can also occur during the second trimester, in which the placenta separates from the uterus too early, and a miscarriage can occur
- During these stages, harmful exposures can cause:
- Growth problems
- Babies may be smaller or larger than average which can put them at risk for health problems
- Minor birth defects
- Functional defects such as learning problems
- Other pregnancy complications
- Premature delivery
- Low levels of amniotic fluid
- Withdrawal symptoms in newborns if mothers use certain medications late in pregnancy
- Growth problems
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