Most obstetricians determine the duration of pregnancy from the time elapsed since the mother's last menstrual period. Of course, this is usually about 2 weeks before ovulation and pregnancy actually occurs. The normal duration of pregnancy is 280 days or 42 weeks from the last menstrual period. Pregnancy is also divided into thirds, or trimesters, which are each about 13 weeks in length.
Fertilization occurs when the male sperm enters the female ovum. Very soon after conception, the early placenta begins to develop. By 4 weeks of gestation, the embryo is about 3/8ths of an inch long. Early fetal eyes and limb buds are present by 1 month once the fetal heart is completely formed by 6 weeks and can be seen on an ultrasound. Blood begins to pump at this time. Also, a ridge of tissue, which will become the fetal brain and spinal cord, runs along the embryo's body. At about 5 weeks, urine pregnancy tests can detect HCG, which is secreted by the placenta. This is the basis of a positive pregnancy test. All early fetuses are female. If testes are present, testosterone, the male hormone, is secreted beginning about 8 weeks and those fetuses become male. The rest remain female. Now we call the baby a "fetus" instead of an embryo. By 10 weeks, all major body organs are present, except for the fetal lungs, which are the last to completely develop. All remaining weeks of gestation are devoted to the growth and maturing of these key body structures. By 12 weeks, the 2-1/2 inch fetus weighs 1-1/2 ounces. The heartbeat can now be heard by listening to the mother's abdomen with a Doppler. The nails and skin are now present. The fetus begins to move spontaneously. Soon after, the kidneys begin to make urine and blood begins to form in the bone marrow.
By 4 months, the growing fetus weights 7 ounces - just about ½ a pound - and 5 inches from head to rump. The fetus can suck, swallow and make early breathing movements. The arms and legs are completely developed. At 18 weeks, all body and facial features are recognizable. The eyes begin to blink. The fetus moves quite a lot now, though the mother may not feel it. At 5 months, or 20 weeks, the halfway point in gestation is reached. Although the primitive lungs cannot breathe air, early respiratory movements begin. The fetus can be seen sucking its thumb on an ultrasound. Soon the fetus begins to hear the mother's heartbeat and voice. The fetus wakes and sleeps. Lanugo, the fine hair covering the fetal body, appears. Some small amounts of scalp hair also begin to grow. The fetal sex can be seen with an ultrasound.
By 28 weeks, the beginning of the third trimester, the fetus can survive outside of the uterus if the lungs are developed enough. The fetus is usually breech (butt or legs) coming into the pelvis first. From 32 weeks onward, the fetus is in a growth phase. The muscles become stronger and the fetus can turn with greater ease. By 36 weeks, 97% of the fetus has turned and become cephalic or head first. At 38 weeks, the fetus is finally term and the lungs are usually mature. The fetus can easily survive outside of the womb. Forty weeks is full term and the average fetus is 20 inches long and weighs 7 pounds. These days, most obstetricians would not let the pregnancy go beyond 42 weeks.