Prenatal Development

• Prenatal development is often divided into three major phases: the period of the zygote, the period of the embryo, and the period of the fetus.
• The period of the zygote lasts from conception through implantation, when the developing zygote becomes firmly attached to the wall of the uterus.
• The period of the zygote normally lasts about 10 to 14 days.
• During the period of the zygote, the fertilized ovum, or zygote, moves down the fallopian tube toward the uterus and divides by mitosis into two cells.
• These two cells and all the resulting cells continue to divide, forming a ball-like structure, or blastocyst, that will contain 60 to 80 cells within 4 days of conception.
• Cell differentiation has already begun during the period of the zygote, with the inner layer of the blastocyst becoming the embryo, and the outer layer of cells developing into tissues that protect and nourish the embryo.
• Once implanted, the blastocyst’s outer layer rapidly forms four major support structures that protect and nourish the developing organism: the amnion, the chorion, the allantois, and the yolk sac.
• The amnion is a watertight sac that fills with fluid from the pregnant woman’s tissues, cushioning the developing organism against blows, regulating its temperature, and providing a weightless environment that will make it easier for the embryo to move.
• The yolk sac, attached to the amnion, produces blood cells until the embryo is capable of producing its own.
• The chorion surrounds the amnion and eventually becomes the lining of the placenta, a multipurpose organ.
• The allantois forms the embryo’s umbilical cord.
• The second phase of prenatal development, the period of the embryo, lasts from the beginning of the third week through the end of the eighth.
• By the end of the third week, the embryonic disk is rapidly differentiating into three cell layers: the outer layer, or ectoderm, will become the nervous system, skin, and hair; the middle layer, or mesoderm, will become the muscles, bones, and circulatory system; the inner layer, or endoderm, will become the digestive system, lungs, urinary tract, and other vital organs such as the pancreas and liver.
• In the third week after conception, a portion of the ectoderm folds into a neural tube that soon becomes the brain and spinal cord.
• Some teratogens cause “sleeper effects” that may not be apparent until later in the child’s life.
• By the end of the fourth week, the heart has not only formed but has already begun to beat.
• Thalidomide is a mild tranquilizer that, taken early in pregnancy, can produce a variety of malformations of the limbs, eyes, ears, and heart.
• Toxoplasmosis is a disease caused by a parasite found in raw meat and cat feces; it can cause birth defects if transmitted to an embryo in the first trimester and miscarriage later in pregnancy.
• Acquired immunodeficiency syndrome (AIDS) is a viral disease that can be transmitted from a mother to her fetus or neonate and that results in a weakening of the body’s immune system and, ultimately, death.
• The longer the exposure to or higher the “dose” of a teratogen, the more likely it is that serious harm will be done.
• Rubella, also known as German measles, is a disease that has little effect on a pregnant woman but may cause a number of serious birth defects in developing organisms who are exposed in the first 3 to 4 months of pregnancy.
• Syphilis is a common sexually transmitted disease that may cross the placental barrier in the middle and later stages of pregnancy, causing miscarriage or serious birth defects.
• Diethylstilbestrol (DES) is a synthetic hormone, formerly prescribed to prevent miscarriage, that can produce cervical cancer in female offspring and genital-tract abnormalities in males.
• Cleft palate is a congenital disorder in which the roof of the mouth does not close properly during embryonic development, resulting in an opening or groove in the roof of the mouth.
• Genital herpes is a sexually transmitted disease that can infect infants at birth, causing blindness, brain damage, or even death.
• Cleft lip is a congenital disorder in which the upper lip has a vertical (or pair of vertical) openings or grooves.
• Embryos and fetuses can be affected by fathers’ as well as by mothers’ exposure to some teratogens.
• Fetal alcohol syndrome (FAS) is a group of serious congenital problems commonly observed in the offspring of mothers who abuse alcohol during pregnancy.
• Fetal alcohol effects (FAE) are a group of mild congenital problems that are sometimes observed in children of mothers who drink sparingly to moderately during pregnancy.
• The embryo’s circulatory system now functions on its own, for the liver and spleen have assumed the task of producing blood cells.
• The fetal heartbeat can easily be heard with a stethoscope, and as the amount of bone and cartilage increases as the skeleton hardens, the skeleton can be detected by ultrasound.
• From 15 or 16 weeks through about 24 or 25 weeks, simple movements of the tongue, lips, pharynx, and larynx increase in complexity and coordination, so that the fetus begins to suck, swallow, munch, hiccup, breathe, cough, and snort, thus preparing itself for extrauterine life.
• The male testes secrete testosterone, the male sex hormone responsible for the development of a penis and scrotum.
• By the end of the second month, the embryo is slightly more than an inch long and weighs less than 1/4th of an ounce. At this point, all the major structures of the human are formed, and the organism is beginning to be recognizable as a human.
• In fact, infants born prematurely may have difficulty breathing and sucking because they exit the womb at an early stage in the development of these skills — simply put, they haven’t had enough time to practice.
• The fetus is now covered by a white, cheesy substance called vernix and a fine layer of body hair called lanugo.
• By the end of the 16th week, the fetus has assumed a distinctly human appearance, although it stands virtually no chance of surviving outside the womb.
• By the end of the third month, the sex of a fetus can be detected by ultrasound and its reproductive system already contains immature ova or sperm cells.
• In the absence of testosterone, female genitalia form.
• During the fifth and sixth months, the nails harden, the skin thickens, and eyebrows, eyelashes, and scalp hair suddenly appear.