Prenatal development covers a 40-week period that includes an astounding amount of growth and change. Starting with the microscopic zygote and ending with a fully formed baby, the phases of child development in the womb encompasses how your infant grows from a few single cells into an embryo and later into the completely complex fetus.
The first trimester includes the time from conception through the 13th week of pregnancy. According to the American Pregnancy Association, between six and 12 days after conception, the fertilized egg will embed itself in the lining of a woman's uterus. This ushers in the embryonic stage with the beginnings of brain, gastrointestinal, heart and spinal cord development. By four to five weeks, the embryo has visible arm and leg buds and a heartbeat, with all organs starting to grow by week seven. At gestational week eight, the embryo is only 1-inch long, but is beginning to develop bones and even facial features. As the first trimester ends, so does the embryonic period. During the final weeks of the first trimester, your baby moves into the fetal stage of development.
During the second three-month stage of pregnancy, the fetus is growing and developing into a more complex human being. The American Pregnancy Association notes that during the beginning of the second trimester, the fetus is starting to make sucking motions and moving around inside of the womb. By the gestational age of 17 to 20 weeks, most moms can feel distinct fetal movements. The following few weeks see the fetus moving into more of a newborn look with body fat developing and full facial features. At the end of the second trimester, the fetus is possibly capable of surviving -- with the assistance of medical technology -- outside of the womb, but is still only roughly 14 inches long with a weight of less than 2 1/2 pounds.
The last, or third, trimester of your pregnancy is a time of refinement. The final steps of finishing fetal development are occurring, and by the end of this period your baby is ready to survive completely out of the womb. During the gestational weeks 27 through 32, the fetus is beginning to make rhythmic lung movements, although your baby can't breathe entirely alone yet. Following 32 weeks gestation, most fetuses start turning into the head down position in preparation for birth. By 38 weeks, the fetus is full term and is developmentally ready to leave the womb. While some babies are born at this time, others might wait until closer to full term -- 40 weeks -- before they are ready for birth.
Medical professionals and parents alike look for some major milestones during the in utero period. While in the womb, one of the first notable marks is hearing the growing baby's heartbeat. Although the heart is starting to form early on in the first trimester, you won't have that magical moment when you can hear your baby's heartbeat for the first time, according to the American Pregnancy Association, until the fetal age of 15 to 18 weeks. Another major milestone is finding out the sex of your baby. The Mayo Clinic notes that the genitalia is developing by week 11, but you most likely won't have the ability to see it on a sonogram until closer to 20 weeks along.