A baby develops in her mother's womb over the course of about 38 weeks--between the time that the mother's egg is fertilized by the father's sperm until the baby is born. The development that takes place prior to birth is called prenatal development. It is a complex process that consists of several, remarkably consistent stages of development.
Calculating a Baby's Prenatal Age
A baby's prenatal age can be described in at least two distinct ways. First, the baby's gestational age states the time that has passed since the first day of the mother's last menstrual cycle prior to pregnancy. Second, a baby's post-fertilization age states the amount of time that has passed since the fertilization of the egg. Because an egg is usually fertilized approximately two weeks after the start of the last menstruation, the gestational age is typically two weeks greater than the post-fertilization age. Doctors make use of both types of age descriptors, often preferring the gestational age, because it can be directly known, whereas the post-fertilization age can only be estimated. The rest of this article will use the post-fertilization age to describe the stages of development.
Zygote Stage (0 to 5 Days)
The egg travels down the fallopian tube of the mother, where it may be fertilized by a sperm. The moment that it is fertilized, it becomes a zygote. The zygote contains DNA from the mother and the father, combined to form the full set of genetic material that will control the cellular production of the new baby. In some cases, two eggs are released in the same menstrual cycle, and they are both fertilized, forming two zygotes. If both zygotes develop, they become fraternal twins. The process of producing identical twins involves only a single zygote, which forms into an embryo before dividing into two separate bodies. As the zygote travels down the fallopian tube and into the uterus, it divides and replicates itself. Once it has formed a ball of cells, it is referred to as a blastocyst.
Blastocyst Stage (5 to 10 Days)
The blastocyst implants itself in the wall of the mother's uterus within a few days of fertilization. There, it continues to grow rapidly, into a ball of several hundred cells. Most of the ball is only one layer of cells thick. This part of the ball becomes the placenta, an organ that serves as a connector to the bloodstream of the mother and helps to mediate development. One region of the blastocyst is three to four cells thick. This region eventually becomes the embryo itself.
Embryonic Stage (2 to 7 Weeks)
The embryo floats in a thin, fluid-filled membrane called the amniotic sac. It is connected to the placenta via the umbilical cord. During this stage of baby development in the womb, the human shape becomes recognizable. The spinal cord and brain are clearly differentiated from the rest of the body. The heart forms and begins to beat. The arms and legs sprout at the beginning of this stage; by the end, fingers and toes form. As the brain develops, it starts to produce electrical activity doctors can record. The nerve impulses produced by the brain begin to cause the muscles to contract and move.
Teratogens are agents that can cause birth defects by disrupting normal development in the womb. Drugs, alcohol, infection and radiation are the most common teratogens. According to the National Institutes of Health, the embryo is especially sensitive to teratogens because of the great amount of developmental change that occurs during this stage. For this reason, expectant mothers are strongly encouraged to avoid exposure to teratogens during this stage of development.
Fetal Stage (8 to 37 Weeks)
This is the final stage of prenatal development. The primary structures of the body are all in place by the start of this stage. Thus, the fetal stage seems to be mostly about filling in small details. For instance, the hands and feet are already in place at the start of the fetal stage. By the end, however, fingernails and toenails appear--around week 20--along with fingerprints and footprints--around week 24. The fetus also grows larger rapidly, until the baby fills the amniotic sac and presses on the mother's belly. The Merck Online Medical Manual says that at this stage, around week 20, the mother begins to feel frequent movements of the growing fetus. Starting around 13 weeks, the fetus begins to grow a wispy type of hair called lanugo, which eventually covers the entire body. Normally, the lanugo vanishes by the time the fetus is born, but some residual lanugo can often be seen on newborns, especially those that are born prematurely. Around week 17, the fetus begins to respond to sound from the outside world, some of which is able to penetrate the womb and reach the ears of the fetus. Beyond week 24, the lungs become able to derive oxygen from air. Under normal circumstances, they will not need to, as the fetus receives all necessary oxygen from the mother's bloodstream, delivered through the placenta and the umbilical cord. The Merck Online Medical Manual says, if the baby is born prematurely beyond week 24, the baby may survive, in part due to the lung function available at this stage.
