During the 40 weeks of pregnancy, as your belt tightens and your belly expands, your baby transforms from a single-celled organism into a fully formed fetus prepared to survive outside the uterus. Although growth and development continue after birth, the first nine months in the uterus provide the essential foundation for a healthy life.
Conception
Your baby begins as a zygote, a one-celled organism formed in the fallopian tubes when the egg and the sperm come together. The cell begins dividing as it travels toward the uterus, forming a tiny cluster of cells called a blastocyst. This cell cluster then nestles into the lining of the uterus, where the embryo begins to develop further. This whole period, the start of the first trimester, generally lasts about two weeks.
First Trimester
After implantation, the embryo's organs and major systems begin to develop. According to the Mayo Clinic, by the end of week six, the embryo's heart is active, pumping blood. As the trimester continues, numerous events occur, including the initial development of arms and legs, facial features and fingernails. The embryo begins to move, though you won't yet feel the movements. At the end of the trimester, week 12, your fetus will be about 2-1/2 inches long.
Second Trimester
During the second trimester, your fetus approximately triples its week-12 length. The fetus begins to create urine, bones form and a layer of fat builds up under the skin to serve as insulation after birth. Fine hair called lanugo begins growing all over the body, and regular hair appears on the head. The genitals develop and can be seen on ultrasound. The lungs are mature, preparing the fetus for life outside the womb.
Third Trimester
During the third trimester, a fetus's eyes open and begin to respond to changes in light, according to the Mayo Clinic. Hair on the head continues to grow, and the lanugo falls off while the skin's white coating called the vernix caseosa, thickens. Also, the fetus practices breathing and begins storing minerals that will help strengthen the bones. In the last four weeks of pregnancy, the fetus's weight gain is rapid: about a half-pound per week.
Birth and Viability
Babies are often born a couple of weeks before or after their due date, but fetuses are considered full-term and ready to survive outside the womb at 37 weeks. With intensive medical care, babies born as early as week 24 can survive. By week 28, a healthy baby's chance of survival outside the womb is 90 percent, according to the Mayo Clinic.
