How Many Weeks Does it Take for a Baby to Develop?

Most people talk about nine months of pregnancy, but medical professionals speak of a typical pregnancy lasting for 40 weeks. Technically, though, it takes about 38 weeks for a human baby to develop in utero from conception to birth.

  1. History

    • The 40-week-pregnancy schedule dates back to the era when conception was thought to happen during the menstrual period. We now know that conception occurs around the time of ovulation, which for many women (but not all) is about two weeks after menstruation begins.

    Pregnancy Stages

    • Medical professionals divide pregnancy into three stages, or trimesters.

    The First Trimester

    • During the first 12 weeks of pregnancy, the fertilized ovum implants itself into the uterine lining (week 3) and develops a spinal cord (week 4) and a beating heart (week 5). All limbs and major organs, as well as the placenta, are in place by 12 weeks' gestation.

    The Second Trimester

    • From week 13 to week 25, the muscles and nervous system develop, and the mother begins to feel fetal movement. Babies born at about 24 weeks' gestation (and sometimes even earlier) can, with help, survive outside the womb.

    The Third Trimester

    • From week 26 until birth, the baby's brain and body grow quickly, and lung development prepares the baby to breathe air. A full-term baby is born in the 40th week of pregnancy, though a variation of a week or two earlier or later does not usually pose a medical problem.

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