The moment the sperm penetrates the ovum is called conception. A fertilized egg is called a zygote. It will reach the uterus 3-4 days later.
4 Weeks
Once the zygote reaches the uterus it is called a embryo. The embryo can float around the uterus for up to 48 hours before implantation. At implantation, the mother and embryo connects to form the placenta.
6 Weeks
The head, mouth, liver, and intestines are beginning to form. The tiny heart is beating. The embryo is 1/5 of an inch in length.
10 Weeks
Facial features, hands, feet, legs, toes, and fingers are distinguishable. Many of the internal organs and the nervous system begins to function. The embryo is approximately 1 inch in length.
14 Weeks
Eyelids, fingernails, and toenails are forming. Muscles and sex organs are also developing. The baby is also moving around the uterus now. The fetus is 3 inches long.
2nd Trimester
Time
Stage Of Development
Hair is beginning to grow on the head and body. The baby can blink, move its mouth, and can grasp. The bones are hardening. The baby is about 5 inches long now.
18 Weeks
22 Weeks
The skin is now opaque instead of transparent and the sweat glands are developing. The fetus is about 10 inches long and weighs approximately 1/2 pound.
26 Weeks
The eyes have completely formed and the tongue has taste buds. The baby can now exhale, inhale, and cry. If the baby were born now it would have over a 50% chance of survival.
3rd Trimester
Stage Of Development
Time
28 Weeks
The eyebrows and eyelashes are visible. The baby is now storing about 2-3% fat. Talk to your baby because it can now recognize your voice. Weight is approximately 2 pounds.
30 Weeks
Fat is continuing to build up under the skin. The head is now in proportion with the rest of the body. Baby is now sensitive to light, sound, smell, and taste. Bone marrow is producing red blood cells.
The baby is about 3 pounds.
32 Weeks
The brain is rapidly growing. If it is a boy, the testes are descending. Approximately 4 pounds now.
34 Weeks
Fat has continued to build and it has caused the skin to turn pink. The baby now has awake and sleeping periods. The toenails and fingernails are completely grown. The immune system is building.
36 Weeks
The baby is about 5 1/2 pounds now. Fat has formed on the arms and legs and has made dimples on the elbows and knees. Baby may drop into the birth canal.
38 Weeks
The baby is practicing breathing and maturing. The baby will turn towards a light. The bowels are accumulating meconium (the first bowel movement). 6 1/2 pounds is the average size now.
40 Weeks
The lanugo is about gone. The average size is 7 1/2 pounds. The baby is ready to be born.
OB/GYNs count the weeks of pregnancy from the first day of the last menstrual cycle. This chart (as well as most others) describes the developing baby by the number of weeks since conception. To determine age of the unborn baby, subtract 2 weeks. Please keep in mind that this info is approximate. Each pregnancy varies and growth rates differ.
