Within hours after conception, the zygote begins duplication and division. First, the 23 pairs of chromosomes carrying all the genes duplicate to form two complete sets of the genome. These two sets move toward opposite sides of the zygote, and the single cell splits neatly down the middle into two cells, each containing the original genetic code. These two cells duplicate and divide, becoming four, which duplicate and divide, becoming eight, and so on.

The name of the developing mass of cells changes as it multiplies—from morula to blastocyst, from embryo to fetus—and finally, at birth, baby. (Developmental Link: Prenatal growth is detailed in Chapter 4.)

Nine months after conception, a newborn has about 26 billion cells, all influenced by whatever nutrients, drugs, hormones, viruses, and so on came from the pregnant woman. Almost every human cell carries a complete copy of the genetic instructions of the one-celled zygote.

Adults have about 37 trillion cells, each with the same 46 chromosomes and the same thousands of genes of the original zygote (Bianconi, et al., 2013). This explains why DNA testing of any body cell, even from a drop of blood or a snip of hair, can identify “the real father,” “the guilty criminal,” “the long-lost brother.” DNA lingers long after death. Some living African Americans claimed Thomas Jefferson as an ancestor: DNA testing proved some of them right and some of them wrong (Foster et al., 1998).

Indeed, because the Y chromosome is passed down to every male descendant, and because the genes on the Y typically do not change much from one generation to another, men have the Y of a male ancestor who died thousands of years ago.

Tracing the Y chromosome suggests that thousands of East Asian men may be descendants of Genghis Khan—although that twelfth-century leader’s bones and thus his DNA have never been found (Stoneking & Delfin, 2010). Such genetic sleuthing is not totally reliable: It is quite possible to trace ancestry for several generations; it is much more complex to go back for hundreds of years. But history suggests Khan may have impregnated many women.

The cells that result from the early duplication and division are called stem cells these cells are able to produce any other cell and thus to become a complete person. Indeed, as later described, sometimes these cells split apart and each becomes an identical twin.

After about the eight-cell stage, although duplication and division continue, a third process, differentiation, begins. In differentiation, cells specialize, taking different forms and reproducing at various rates depending on where they are located. For instance, some cells become part of an eye, others part of a finger, still others part of the brain. They are no longer stem cells. Blood cells in the umbilical cord, however, may act like stem cells.

Scientists have discovered ways to add genes to certain differentiated cells in a laboratory process that reprograms those cells, making them like stem cells again. However, scientists do not yet know how to use reprogrammed stem cells to cure genetic conditions without harming other cells. In fact, there are many kinds of stem cells, and thousands of potential uses (Slack, 2012).

One use of reprogrammed cells has been found: to test drugs to treat diseases caused by genes, either directly (such as sickle-cell anemia) or indirectly (such as heart disease, diabetes, and dementia) (Zhu & Huangfu, 2013; Vogel, 2010).

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Some restrictions on stem cell research in the United States were lifted in 2009, and some states (e.g., California) and nations (e.g., South Korea) allow more extensive research, but everywhere many ethical and practical issues remain (Nguyen et al., 2013). As the head of the Michael J. Fox Foundation for Parkinson’s Research said, “All my exposure was pop media. I thought it was all about stem cells. I have not totally lost hope on cell replacement, I just don’t think it’s a near-term hope” (Hood, quoted in Holden, 2009).

One of the first signs of differentiation is the formation of the placenta, the organ that sustains the developing person throughout pregnancy. The outer cells of the blastocyst surround the inner cells, even before differentiation of body cells has begun, soon creating tiny blood vessels called villi that will connect the mother to the embryo via the umbilical cord. Those blood vessels exchange waste products from the fetus and nourishment from the mother to enable growth and life.

In some cultures, the placenta is revered and ceremoniously buried once the baby is born. In the United States it is usually discarded, “a throwaway organ” that deserves more respect (Kaiser, 2014, p. 1073).

The fact that the placenta is formed from the early duplicating cells allows early genetic testing. In chorionic villus sample (CVS), a sample of the placental blood is removed and analyzed. Many chromosomal and genetic disorders can be detected at this early stage.

Later in pregnancy, cells from the fetus are sloughed off and circulate in the amniotic fluid, and these can also be removed and analyzed in a process called amniocentesis. (Developmental Link: Chromosomal and genetic disorders are discussed at the end of this chapter.)

Thus far we have described conception as if one sperm fertilized one ovum, beginning the process of duplication and differentiation. But that is not always the case. To understand multiple conceptions, you need to know the difference between monozygotic and dizygotic twins (see Visualizing Development, p. 76).

Although every zygote is genetically unique, about once in 250 human conceptions duplication results in one or more complete splits, creating two, or four, or even eight separate zygotes, each identical to the first single cell. This creation of identical embryos has been done experimentally for lower animals, but it is illegal for humans in a laboratory. However, nature does it occasionally in the first hours after conception. (An incomplete split creates conjoined twins, formerly called Siamese twins.)

If each of those separated cells from one zygote then duplicates, divides, differentiates, implants, grows, and survives, multiple births occur. One separation results in monozygotic (MZ) twins, from one (mono) zygote (also called identical twins). Two or three separations create monozygotic quadruplets or octuplets.

Because monozygotic multiples originate from the same zygote, they have identical genetic instructions for appearance, psychological traits, disease vulnerability, and everything else genetic. Remember however, that epigenetic influences begin as soon as conception occurs: Monozygotic twins look very much alike, but their environment can influence their development in many ways that distinguish one from the other.

Monozygotic twins are blessed in some ways: They can donate a kidney or other organ to their twin with no organ rejection, thus avoiding major complications with surgical transplants. On a lighter note, they can also befuddle their parents and teachers, who may use special signs (such as different earrings) to tell them apart. Usually, the twins themselves find their own identities while enjoying twinship. They might enjoy inherited athletic ability, for instance, with one playing basketball and the other soccer.

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As one monozygotic twin writes:

Twins put into high relief the central challenge for all of us: self-definition. How do we each plant our stake in the ground, decide how sensitive, callous, ambitious, cautious, or conciliatory we want to be every day? … Twins come with a built-in constant comparison, but defining oneself against one’s twin is just an amped-up version of every person’s life-long challenge: to individuate, to create a distinctive persona in the world.

[Pogrebin, 2010, p. 9]

Among naturally conceived twins, only about one in five pairs is monozygotic. About once in 60 conceptions, dizygotic (DZ) twins are conceived. They are also called fraternal twins, although since fraternal means “brotherly” (as in fraternity), the term is inaccurate. DZ twins are as likely to be two girls, or a boy and a girl, as two boys. In every case, dizygotic twins began life as two separate zygotes created by two ova fertilized by two sperm at the same time. (Usually, women release only one ovum per month, but sometimes double or triple ovulation occurs, a tendency that is affected by genes.)

People sometimes say that twinning “skips a generation” but that also is not accurate. What it skips is fathers—but not exactly. Since dizygotic twinning depends on multiple ovulation, the likelihood of a woman bearing twins depends on her genes, not her husband’s. However, a man has half his genes from his mother. If multiple ovulation is in his family, he will not produce twins, but if the X he inherited from his mother is the one that encouraged multiple ovulation, his daughters (who always get his X) might.

When dizygotic twinning occurs naturally, the incidence varies by ethnicity. For example, about 1 in 11 Yorubas in Nigeria is a twin, as are about 1 in 45 European Americans, 1 in 75 Japanese and Koreans, and 1 in 150 Chinese. Age matters, too: Older women more often double-ovulate and thus have more twins.

After twins are conceived, their chance of survival until birth depends on the prenatal circumstances. An early sonogram might reveal two developing organisms, but later only one embryo continues to grow. This vanishing twin phenomenon may occur in about 12 percent of pregnancies (Giuffrè et al., 2012).

Like all full siblings, DZ twins have about half of their genes in common. They can differ markedly in appearance, or they can look so much alike that only genetic tests can determine whether they are monozygotic or dizygotic.

Chance determines which sperm fertilizes each ovum, so about half are same-sex pairs and half are boy–girl pairs. Of course, in the rare incidence that a woman releases two ova at once and also has sex with two different men over a short period, it is possible for fraternal twins to have different fathers and thus share only a fourth of the same genes.

The statistic above—twins born about once in 60 pregnancies—refers to natural conception. However, the twin birth rate has increased by 50 percent in the past two decades, and the triplet birth rate has doubled. The primary reason is that when couples want children but are infertile (a condition affecting about 12 percent of U.S. couples), about half of them choose assisted reproductive technology (ART).

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Infertility is defined as failure to conceive a child after a year of trying. Rates vary by nation, primarily because of the quality of medical care and the incidence of sexually transmitted infections, many of which cause infertility. About one-third of all fertility problems originate in the woman, about one-third in the man, and in about one-third, the cause is unknown.

Some couples are subfertile, that is, less fertile than ideal but not sterile. It is not unusual for a couple to believe they are infertile, begin ART or adoption, and then spontaneously conceive. For subfertile couples, one developmental factor falls more heavily on women. Although both sexes become less fertile every year beginning at about age 18, for women the decline is steeper. Pregnancy after age 40 is unusual, and menopause (naturally at about age 50) makes ovulation impossible. By contrast, men produce sperm lifelong.

Since age is a factor, if a couple have trouble conceiving, they should consult a fertility specialist as soon as they can. The simplest cause is that the ovaries do not release ova. In that case, a woman can take a drug (usually Clomid) to cause ovulation. The drug may cause release of several ova at once, causing multiple births and risking preterm birth—a risk many are willing to take.

Another relatively simple solution appears when the man produces no viable sperm. In that case, the woman can be inseminated with another man’s sperm, a procedure that usually does not include any personal contact between the man who donates sperm (stored in a laboratory) and the woman. Doctors have used this method of fertilizing an ovum for more than 100 years.

In the twenty-first century, the most common ART method is in vitro fertilization (IVF), “a relatively routine way to have children, with an estimated 5 million babies born to date” (Thompson, 2014, p. 361). In this method, ova are surgically removed from an ovary, fertilized in a lab dish (in vitro means “in glass”), and then inserted into the uterus.

Often, a sperm is inserted directly into each ovum to improve the odds of fertilization, in a procedure called intra-cytoplasmic sperm injection (ICSI). Zygotes that fail to duplicate, or blastocysts tested positive for serious genetic diseases, are rejected. Many women ask that several fertilized ova be inserted: twins or even triplets seem like a bonus when a couple has been infertile for years.

Doctors have refined this procedure since the first IVF baby, Louise Brown, was born in England in 1978. Now they wait two to six days after conception, making sure division and duplication occur smoothly, before inserting the developing cells into the uterus. Between 1 and 3 percent of all newborns in developed nations and thousands more in developing nations are the result of IVF, making one scholar call IVF “a new norm of family life” (Franklin, 2013, p. 30).

Those numbers allow careful, longitudinal research. In general, IVF children develop as well as other children. A slightly higher risk of birth complications is evident, but that might not be directly from the IVF procedure but rather from other conditions common in such pregnancies: older parents and low birthweight (Fauser et al., 2014).

ART has enabled millions of couples to have children. Indeed, some parents have children who are not genetically or biologically theirs if others donate the sperm, the ova, and/or the womb. The word donate may be misleading, since most donors—often college students—are paid for their sperm, ova, or pregnancy.

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The reality that three people could contribute to the birth of a baby before someone else becomes the child’s parent evokes moral issues. Increasingly, the “real” parents are thought to be those who raise a child, not those who conceive it, although not everyone agrees with this view (Franklin, 2013). Whether with their own gametes or someone else’s, most people who become parents via IVF are married heterosexuals, but ART makes parenthood possible for many others, including single parents and same-sex couples.

Some nations forbid IVF to many who wish it, asking not only for proof of marriage but sometimes proof of financial and emotional health (as in Switzerland). Several European nations limit the numbers of blastocysts inserted into the uterus at one time, partly because national health care pays for both IVF and newborn care. According to research in seven nations, inserting just one blastocyst results in as many successful pregnancies as inserting multiple blastocysts, with fewer low-birthweight newborns (7 percent compared to 30 percent), primarily because each fetus develops alone (Grady et al., 2012).

The United States has no legal restrictions on IVF, although the procedure is expensive (about $20,000 for all the drugs and monitoring as well as the procedure itself) and not covered by insurance, which makes it beyond the reach of many couples. Medical societies provide some oversight. For example, the California Medical Board removed his license to practice medicine from the physician who inserted 12 blastocysts in Nadya Suleman. She gave birth to eight surviving babies in 2009, a medical miracle but a developmental disaster.