First Trimester

2 weeks - Fertilization: the sperm and egg join in the fallopian tube to form a unique human being. Forty-six chromosomes combine, which pre-determine all of a person's physical characteristics.

The picture on the right is a fertilized egg, only thirty hours after conception. Magnified here, it is no larger than the head of a pin. Still rapidly dividing, the developing embryo, called a zygote at this stage, floats down from the fallopian tube and towards the uterus.

3 weeks - Once in the uterus, the developing baby, called a blastocyst, searches for a nice place to implant, where it actually burrows beneath the surface of the uterus. The yolk sac, shown on the left, produces blood cells during the early weeks of life. The unborn baby is only one-sixth of an inch long, but is rapidly developing. The backbone, spinal column, and nervous system are forming. The kidneys, liver, and intestines are taking shape.

4 weeks - The baby produces hormones which stop the mother's menstrual cycle.

5 weeks - Baby is the size of a raisin. By day twenty-one, the baby's tiny heart has begun beating. The neural tube enlarges into three parts, soon to become a very complex brain. The placenta begins functioning. The spine and spinal cord grows faster than the rest of the body at this stage and give the appearance of a tail. This disappears as the baby continues to grow.

7 weeks - Facial features are visible, including a mouth and tongue. The eyes have a retina and lens. The major muscle system is developed, and the unborn baby practices moving. The baby has its own blood type, distinct from the mother's. These blood cells are produced by the liver now instead of the yolk sac.

8 weeks - The unborn baby, called a fetus at this stage, is about half an inch long. The tiny person is protected by the amnionic sac, filled with fluid. Inside, the baby swims and moves gracefully. The arms and legs have lengthened, and fingers can be seen. The toes will develop in the next few days. Brain waves can be measured.

10 weeks - The heart is almost completely developed and very much resembles that of a newborn baby. An opening the atrium of the heart and the presence of a bypass valve divert much of the blood away from the lungs, as the child's blood is oxygenated through the placenta. Twenty tiny baby teeth are forming in the gums.

12 weeks - Vocal chords are complete, and the baby can and does sometimes cry (silently). The brain is fully formed, and the baby can feel pain. The fetus may even suck his thumb. The eyelids now cover the eyes, and will remain shut until the seventh month to protect the delicate optical nerve fibers.