A single fertilized egg matures into a full-term fetus over the course of approximately 38 weeks. This development is remarkably complex, as a newborn infant emerges from the womb with essentially a full set body parts already formed. The growth process follows a very consistent set of stages.
Zygote Stage (0 to 5 days)
The mother's egg is fertilized by the father's sperm as it travels down one of her fallopian tubes. This fertilized egg is called a zygote. In some cases, two eggs are released in the same menstrual cycle. If both are fertilized and both develop, fraternal twins will be produced. Identical twins are produced when a single zygote is produced that divides into two identical bodies during the embryonic stage. The zygote travels the rest of the way down the fallopian tube and into the uterus, where it implants itself in the lining of the uterine wall.
Blastocyst Stage (5 to 10 days)
The zygote divides and replicates itself, rapidly forming a ball of several hundred cells called the blastocyst. It is only one layer of cells thick in all but one region, where it is three- to four-cells thick. The thick portion of the blastocyst will become the placenta, an organ that provides a connection to the mother and helps to mediate development. The thick section will become the embryo.
Embryonic Stage (2 to 7 weeks)
A part of the placenta forms a transparent, fluid filled membrane called the amniotic sac. The embryo floats in this fluid, attached to the placenta via the umbilical cord. The main parts of the body all form during the embryonic phase. The brain and spinal cord differentiate themselves from the rest of the body. The heart forms and begins to beat. Arms and legs sprout and begin to grow near the beginning of this stage. By the end of the stage, fingers and toes are formed as well. The embryo is very susceptible to damage from alcohol, drugs, radiation, and infection---as a group these are referred to as teratogens. It is a good idea for an expectant mother to avoid teratogens throughout the pregnancy, but, due to the rapid anatomical development of the embryo, it is especially important for her to avoid them during this stage.
Fetal Stage (8 to 37 weeks)
By the start of the fetal stage, all of the main body parts and organs are present. During this stage, the fetus grows greatly in size while the various body parts undergo a finishing process. In week 8, the eyelids close to cover the developing eyes of the fetus. They remain closed until week 28, when they open and begin to periodically blink open and closed. During week 13, a fine hair called lanugo begins to sprout and eventually produce a coat that covers the entire body. This will typically be shed prior to birth, but some may remain at that time, especially for premature infants.
The fetus's genitals become differentiated around week 16, according to the National Institutes of Health, enabling a determination of the sex of the fetus via an ultrasound scan. Around week 17, the heartbeat of the infant can first be heard using a stethoscope. It should be noted that this can be difficult, since the heart moves as the fetus changes its position within the womb, and the heartbeat of the mother is still much louder. Also around week 17, the fetus begins to respond to sound. Beyond this stage, it seems that the fetus can hear noises that are loud enough to penetrate the womb. In week 20, fingernails and toenails begin to grow. They will reach the end of the fingers and toes by week 37.
At approximately 20 weeks, according to the Merck Online Medical Manual, the fetus grows to fill the amniotic sac. As it does, the fetal body movements---especially kicking---can begin to be felt by the expectant mother. In the 24th week, according to the National Institutes of Health, the lungs have developed sufficiently to derive oxygen from air. While the baby is within the womb, all of the necessary oxygen is delivered from the mother's bloodstream via the umbilical cord. If the baby is born prematurely, however, the emerging lung function will provide it with a chance of survival.
