What Week Is Most Critical in 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

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