Many natural changes occur in the mother’s body during embryonic and fetal development. In some cases, the mother’s condition affects the child’s development and growth. In other cases, the physiologic changes in the mother can initiate disease or aggravate preexisting conditions. Additional information is available in any obstetrics textbook. Infertility is discussed in Chapter 19 of this text.

Conception or fertilization of the ovum by a sperm takes place in the oviduct or fallopian tube. During the next few hours, the genetic information contained in the ovum (oocyte) is merged with that contained in the sperm to form the zygote (fertilized ovum), and many mitotic divisions occur as the zygote moves along the fallopian tube toward the uterus. Implantation of the zygote in the uterine wall is completed approx‑imately 1 week after fertilization, and differentiation (specialization) of cells is apparent as the inner cell mass and trophoblast or outer cell mass form. The inner cell mass becomes the actual fetus, whereas the outer cell mass gives rise to the embryonic membranes, the amnion and placenta. The period from 3 to 8 weeks is termed the embryonic stage, and this is a critical time in the development of all the organs and structures in the fetal body. During this period termed organogenesis, cells divide rapidly, move, and differentiate to form the basic functional elements of the various organs systems and external structures such as the limbs and eyes. By the end of 8 weeks, all organs are formed. For example, the primitive fetal heart is beating at 4 weeks. Note all times relate to that which has elapsed since fertilization, thus the CNS is forming within 1 week of the woman’s first missed menstrual period.

Exposure of the embryo to any teratogen (any substance or situation that causes a developmental abnormality) during this early stage usually causes major widespread damage to the developing structures and leads to serious congenital abnormalities (see Fig. 21-6, which outlines the effects of teratogens on the organs at various times during the pregnancy). Common teratogens include drugs, viruses, alcohol, and radiation. It is preferable to avoid all medications, including herbal remedies and those available without prescription during pregnancy or to consult with a physician to determine a safe alternative. Viruses such as rubella (German measles) and erythema infectiosum (fifth disease) are known to cause damage to the embryo and fetus. For example, during the first trimester, maternal rubella affects infants in 90% of cases, causing spontaneous abortion (loss of the embryo or fetus) or major congenital anomalies.

Major abnormalities rarely occur from exposure after 20 weeks. Erythema infectiosum, acquired during the first half of pregnancy, causes severe anemia in the fetus and possible death. Cigarette smoking by the mother usually results in a child with low birth weight and increased irritability and may also result in stillbirth. There is also an increased risk of placenta previa and abruptio placentae with exposure to tobacco. Because alcohol can easily pass through the placental barrier, there is risk of damage to the fetus during the entire pregnancy. Fetal alcohol syndrome, which varies in severity, impairs a child’s neurologic and intellectual development as well as causing unique physical characteristics (e.g., typical facies) and growth retardation. Increased intake of folic acid before and during pregnancy has greatly reduced the incidence of neural tube defects such as spina bifida and anencephaly (see Chapter 14 and Figure 14-26 for information on spina bifida).

After 8 weeks, the term fetus is used and most organs have completed basic formation. Teratogens have less effect on development during this period because cell damage occurs primarily in certain tissues that are actively differentiating at the time of exposure. Continued growth and development result in completion of many specialized structures such as the lungs. Elementary functions can be observed as the limbs move and amniotic fluid is swallowed. However, functional impairment, particularly in the central nervous system, can occur with exposure to teratogens at this stage of development. During the last trimester in utero, the fetus gains weight, and organs such as the lungs mature. With improvements in technology and neonatal care, the fetus may be able to survive (remain viable) outside the uterus as early as 22 to 23 weeks after conception. Birth at such a premature age is often accompanied by complex medical problems as the child adapts to live outside the uterus.

Monozygotic or identical twins form when the developing embryo divides to form two separate, genetically identical embryos. This occurs in approximately 1 : 100 births for unknown reasons. Dizygotic or fraternal twins form when two ova are fertilized by two different sperm, resulting in two genetically dissimilar embryos.