After the mother's egg cell is fertilized in one of the fallopian tubes, it embeds itself in utero--meaning, in the uterus--where it develops into a full-term baby over approximately 38 weeks. This developmental process is impressively consistent and complex, with full textbooks devoted to describing the process. Several fundamental stages of baby development occur in utero.
Zygote Stage (0 -- 5 days)
As soon as the egg is fertilized, it is referred to as a zygote. It contains a single full set of DNA that has been constructed from half of the mother's DNA and half of the father's. In some cases, two eggs are released in the same menstrual cycle. If both of these eggs are fertilized--by two separate sperm--and if both develop, they will become fraternal twins. Identical twins result when a single zygote forms that later separates into two separate bodies during the embryonic stage.
Blastocyst Stage (5 -- 10 days)
The zygote divides and replicates itself rapidly as it travels down the fallopian tube and into the uterus. Within about 10 days, the zygote has become a ball of several hundred cells called the blastocyst. The blastocyst implants itself into the lining of the uterus and continues to grow. It is only one layer of cells thick in all but one area, where it is three to four cells thick. The thin section 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 - 7 weeks)
The main parts of the body emerge during the embryonic stage. The spinal cord and brain begin to form. The heart takes shape and begins to beat. The embryo sprouts arms and legs, which eventually sprout fingers and toes. During this developmental stage, because of the rapid anatomical development, the embryo is very susceptible to damage from teratogens--substances such as alcohol, drugs, radiation and infection. It is always good for an expectant mother to avoid teratogens, but it is especially important during the embryonic stage.
Fetal Stage (8 - 37 weeks)
A range of additional development occurs during the fetal stage. Near the start of this stage, the eyelids close to cover the developing eyes. They stay closed until week 28, after which they open and begin to periodically blink open and shut. Around week 13, a fine hair called lanugo begins to develop and eventually covers the entire body. The lanugo will typically be shed by the time the fetus is born, but some may remain, especially for babies born prematurely. According to the National Institutes of Health, the fetus's genitals differentiate around week 16, enabling a determination of whether the fetus is male or female in an ultrasound scan. Around week 17, the sound of the heartbeat first becomes loud enough to be heard with a stethoscope---although this is still difficult because the mother's heartbeat is much louder. According to the National Institutes of Health, the fetus begins to respond to sound during week 17. Beyond this stage, it seems that the fetus can hear noises that are loud enough to penetrate the womb. Fingernails and toenails become apparent in week 20. Close to week 20, according to the Merck Online Medical Manual, the fetus grows to fill the amniotic sac. As it does, the expectant mother can feel the baby kicking and pushing. During week 24, the lungs develop the ability to process oxygen. 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 be necessary for survival.
