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The First Step: Fertilization

Every culture cherishes pregnant women. Some build in rituals to announce the baby after a certain point during pregnancy, and many use charms to ward off fetal harm. Pregnancy is a time of intense mixed emotions—joyous expectations coupled with uneasy fears.

The female reproductive system includes the uterus and its neck, the cervix; the fallopian tubes; and the ovaries, housing the ova. To promote fertilization, the optimum time for intercourse is when the egg is released. Hormones program ovulation and all of the events of pregnancy. At intercourse, hundreds of millions of sperm, produced in the testes, are ejaculated, but only a small fraction make their way to the fallopian tubes to reach the ovum. When the single victorious sperm penetrates the ovum, the two 23 chromosome pairs (composed of DNA, segmented into genes) unite to regain the normal complement of 46 that form our body’s cells.

During the first stage of pregnancy, the two-week-long germinal phase, the rapidly dividing zygote travels to the uterus, becomes a blastocyst, and faces the next challenge—implantation. The second stage of pregnancy, the embryonic stage, begins after implantation and ends around week 8. During this intense six-week period, the neural tube forms and all the major body structures are constructed—according to the proximodistal, cephalocaudal, and mass-to-specific principles of development.

During the third stage of pregnancy, the fetal stage, development is slower paced. The hallmarks of this stage are enormous body growth and construction of the brain as the neurons migrate to the top of the tube and differentiate. Another defining landmark of this seven-month phase occurs around week 22, when the fetus can possibly be viable, that is, survive outside the womb if born.

The nine months of gestation, or pregnancy, are divided into trimesters. The first trimester is often characterized by unpleasant symptoms, such as morning sickness, and a relatively high risk of miscarriage. The landmarks of the second trimester are looking clearly pregnant, experiencing quickening, and often feeling intensely emotionally connected to the child. During the third trimester, the woman’s uterus gets very large, and she anxiously awaits the birth.

The emotional experience of being pregnant varies, depending on socioeconomic status and, most importantly, social support. To really enjoy her pregnancy, a woman needs to feel cared about and loved. Fathers, the neglected pregnancy partners, also feel bonded to their babies too.

Rarely babies are born with a birth defect. One cause is teratogens, toxins from the outside that exert their damage during the sensitive period for the development of a particular body part.

In general, the embryonic stage is the time of greatest vulnerability, although toxins can affect the developing brain during the second and third trimesters also, producing developmental disorders. While there is typically a threshold level beyond which damage can occur, teratogens have unpredictable effects. Damage may not show up until decades later.

Any recreational drug is potentially teratogenic. Smoking during pregnancy is a risk factor for having a smaller-than-optimal-size baby. Drinking excessively during pregnancy can produce fetal alcohol syndrome, or fetal alcohol spectrum disorder. If the woman has poor coping abilities, stress during pregnancy can produce premature labor. Fetal programming research suggests that societal upheavals experienced during pregnancy can have enduring effects, by producing small babies and promoting weight gain and premature, age-related chronic diseases.

The second major cause of prenatal problems is genuinely “genetic”—chromosomal problems and single-gene diseases. Down syndrome is one of the few disorders in which babies born with an abnormal number of chromosomes survive. Although Down syndrome, caused by having an extra chromosome on pair 21, produces intellectual disability and other health problems, people with this condition do live fulfilling lives.

With single-gene disorders, a specific gene passed down from one’s parents, causes the disease. In dominant disorders, a person who harbors a single copy of the gene gets ill, and each child born to this couple (one of whom has the disease) has a fifty-fifty chance of developing the condition. If the disorder is recessive, both parents carry a single copy of the “problem gene” that is not expressed in real life, but they have a 1-in-4 chance of giving birth to a child with that disease (that is, a son or daughter with two copies of the gene). With sex-linked disorders, the problem gene is recessive and lies on the X chromosome. If a mother carries a single copy of the gene, her daughters are spared (because they have two Xs), but each male baby has a fifty-fifty risk of getting the disease. Through advances in genetic testing, couples (and individuals) can find out if they harbor the genes for many diseases. Genetic testing poses difficult issues with regard to whether people want to find out if they have incurable adult-onset diseases.

Couples at high risk for having a baby with a single-gene disorder (or any couple) may undergo genetic counseling to decide whether they should try to have a child. During pregnancy, tests, including the ultrasound, and more invasive procedures such as chorionic villus sampling (during the first trimester) and amniocentesis (during the second trimester) allow us to determine the baby’s genetic fate.

Infertility can be emotionally traumatic and socially isolating, especially for women because of their historic imperative to bear children. Problems getting pregnant are far from rare today, especially at older ages. The most radical intervention, assisted reproductive technologies (ART), such as in vitro fertilization (IVF), in which the egg is fertilized outside of the womb, is emotionally and physically demanding, costly, and offers no guarantee of having a baby. However, this landmark procedure has given couples who could never have conceived the chance to have a biological child.

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Labor and birth consist of three stages. During the first stage of labor, contractions cause the cervix to efface and fully dilate. During the second stage, birth, the baby emerges. During the third stage, the placenta and supporting structures are expelled.

For most of history, childbirth was life-threatening to both the mother and the child. During the first third of the twentieth century, birth became much safer. This victory set the stage for the later-twentieth-century natural childbirth movement. Today women in the developed world can choose from a variety of birth options, including cesarean sections. Impoverished, developing-world women do not have this kind of access or luxury of choices. Their main concern is surviving the baby’s birth.

After birth, the Apgar scale and other tests are used to assess the baby’s health. While most babies are healthy, low birth weight can compromise development. Very-low-birth-weight infants are most apt to have enduring problems and need careful monitoring in the neonatal intensive care unit during their early weeks or months of life.

Infant mortality is a serious concern in the developing world. While rates of infant mortality are generally very low in developed world countries, the United States has a comparatively dismal standing compared to other affluent countries on this basic health parameter. Even though the environment in the womb (stress during pregnancy) can affect the baby, providing a high-quality environment shapes development at every life stage.