Heredity, Prenatal Development, and Birth

This is Unit Two, part two, on conception, the three prenatal periods, and the agents that can harm development before birth. Part one closed on a gene whose effect depends on the environment. Part two opens with genotypes sensitive to good and bad settings alike, then follows a pregnancy from conception. Carry one question through this part: when a harm reaches a developing child, what decides how much damage it does? The prenatal periods set up the answer, because vulnerability changes across them. At the entry on how teratogens work, the part gives the answer: timing first, then dose, genes, and the care that follows birth. By the end you will be able to explain differential susceptibility. To put the three prenatal periods in order with their spans, and tell them from trimesters. To tell a miscarriage from a stillbirth. To explain what decides whether a teratogen causes harm. And to name infectious, drug and environmental teratogens.

Two entries on genotypes that respond to their surroundings, and on children who come to choose their own.

Genotypes sensitive to bad and good settings alike.

HGD 0172 Differential Susceptibility

Differential susceptibility is the finding that some genotypes are more sensitive than others to environments, both bad and good. In one study of adolescent males, those carrying the short allele of a serotonin-related gene had more depressive episodes when family support was low, and fewer when it was high; those with the long allele showed no such difference. The same genotype was a risk in one family and an advantage in another. So sensitivity cuts both ways. It is a finding about groups, and it cannot tell how any one child will respond.

How children come to choose their environments.

HGD 0173 Niche-Picking

Niche-picking is active gene–environment correlation: people seek out environments that fit their inherited tendencies, as a musical teenager chooses band practice over the drum set her parents once bought. It grows with age. Passive correlation is strongest in infancy and early childhood, evocative correlation stays steady, and active correlation increases as children gain control.

Three entries on reproduction: the organs that make gametes, the monthly cycle, and conception itself.

The organs that make eggs and sperm.

HGD 0174 Gonads and Gametes

Both sexes have gonads that make gametes: ovaries in females produce eggs, and testes in males produce sperm and the hormone testosterone. The female structures include the fallopian tubes, uterus, cervix and vagina; the male, the epididymis, vas deferens and urethra. Sperm are stored in the epididymis until mature, then travel through the vas deferens.

The monthly cycle.

HGD 0175 The Menstrual Cycle

The menstrual cycle averages about twenty-eight days across four phases: menstruation, the follicular phase, ovulation and the luteal phase. Ovulation, the release of an egg, typically comes around days fourteen to sixteen. Cycles vary widely between people. If no fertilisation occurs, the thickened uterine lining is shed and the cycle begins again.

Conception, and the hormones behind it.

HGD 0176 Conception

Conception is the union of a sperm and an egg. Luteinizing hormone triggers the egg's release, and progesterone prepares the uterine lining. Sperm can survive up to five days, while an egg is open to fertilisation for about twelve hours. Only one sperm penetrates the egg, which then becomes impenetrable to the rest.

Three entries on what happens when conception does not come easily, and on the other routes to a family.

When conception does not happen.

HGD 0177 Infertility

Infertility is the failure to conceive after at least one year of trying, and the WHO estimates that one in six people face it at some point: about fifteen percent of females and eleven to twelve percent of males. Nearly half of male infertility has no known cause. Polycystic ovary syndrome is the most frequent female cause.

Medical help with conception.

HGD 0178 Assisted Reproduction

Assisted reproductive technology includes ovulation induction; intrauterine insemination, which places sperm in the uterus; and in vitro fertilization, conception in a laboratory dish, with the embryo then transferred to the uterus. Its risks include multiple births and higher rates of preterm birth, low birth weight and birth defects.

Other routes to parenthood.

HGD 0179 Other Paths to Parenthood

Surrogacy, adoption and foster care are further routes to parenthood alongside medical treatment, and adoption itself can be through an agency, within a family, or international. Access is unequal. The US performs half as many assisted-reproduction procedures as Europe, at the highest fees, and some countries set age limits.

Five entries on the first eight weeks: two timelines, the germinal period, twins, and the embryo's organs.

Two ways of counting a pregnancy.

HGD 0180 Trimesters and Periods

Pregnancy is counted in three trimesters, but prenatal development runs through three periods of its own: germinal, embryonic and fetal. Think of one pregnancy on two timelines, trimesters for the calendar and periods for development. The two do not line up. The germinal and embryonic periods are both over well before the first trimester ends.

The first period, from conception.

HGD 0181 The Germinal Period

The germinal period runs from conception to about two weeks. The zygote, the single cell formed at conception, begins dividing within a day, and about a week after conception a blastocyst of a few hundred cells begins to implant in the uterine wall. An estimated fifteen percent of pregnancies are lost before or during implantation.

How the two kinds of twins form.

HGD 0182 Fraternal and Identical Twins

Dizygotic, or fraternal, twins come from two eggs fertilised at once; monozygotic, or identical, twins come from one zygote that splits. Identical twins arise naturally in about zero point four percent of pregnancies. In vitro fertilization raises the chance, to between about one and a half and five and a half percent.

The embryonic period and its support structures.

HGD 0183 The Embryonic Period

The embryonic period runs from about week three to week eight, when the embryo's support structures form alongside its organs: the amnion's fluid-filled sac, the placenta, and the umbilical cord with its three blood vessels. The placenta handles respiration, nourishment and waste removal, and the chorion becomes its lining.

How the organs form.

HGD 0184 Organogenesis

Organogenesis is the formation of organs. About three weeks after conception, the embryo differentiates into three cell layers: ectoderm, which builds the nervous system and skin; mesoderm, muscle, bone and blood vessels; and endoderm, the gut and lungs. Because organs form here, harm during the embryonic period tends to affect structure most. Hold that.

Four entries: the fetal period, the directions growth takes, all three periods together, and the point of viability.

The last seven months.

HGD 0185 The Fetal Period

The fetal period fills the last seven months of pregnancy, a time of growth and refinement of structures already formed. Lanugo, fine body hair, and vernix, a waxy protective coating, appear around seventeen to twenty weeks, and both usually disappear by about thirty-three to thirty-six weeks after conception.

The directions that growth takes.

HGD 0186 Directions of Growth

Growth follows set directions: cephalocaudal, from head to feet; proximodistal, from the centre outward; and mass-to-specific, from large movements to fine. So arm control arrives before finger control, and the head is large before the legs catch up. These directions are exam vocabulary; each names where growth starts and where it spreads.

All three periods, side by side.

HGD 0187 Three Prenatal Periods

Prenatal development runs through three periods. The germinal period, conception to about two weeks, is the shortest: the zygote divides, becomes a blastocyst and implants. The embryonic period, about week three to week eight, is when the support structures and every major organ form. The fetal period, the final seven months, is for growth and refinement. The periods matter because vulnerability changes across them. Harm in the embryonic period tends to affect body structure most; by the fetal period most organs are less vulnerable, with one exception: the nervous system stays susceptible throughout. Exam questions often name a week and ask which period it falls in.

When a fetus can survive outside the womb.

HGD 0188 The Age of Viability

The age of viability is the earliest point at which a prematurely born fetus may survive, usually only with intensive care: about twenty-two to twenty-five weeks. Viability varies with a country's intensive-care capacity. Preterm means born before thirty-seven weeks, a much later line. Keep the two lines apart.

Two entries on pregnancy loss, and the line at twenty weeks that divides the two words for it.

Loss in the first twenty weeks.

HGD 0189 Miscarriage

Miscarriage, or spontaneous abortion, is pregnancy loss in the first twenty weeks. It is the most common complication, ending about fifteen percent of pregnancies, and early losses are often traced to chromosomal abnormalities in the embryo itself. The misconception is that the mother caused it. Most miscarriages occur for reasons outside the birth mother's control.

Loss later in pregnancy.

HGD 0190 Stillbirth

In the United States, a stillbirth is the loss of a pregnancy at or after the twentieth week, the point where miscarriage ends; the same loss is labelled differently on either side of that line. International comparisons often use a later threshold of twenty-eight weeks, so rates reported across countries are not always comparable.

Six entries on teratogens: what they are, what decides their harm, and the three kinds. The opening question is answered here.

Agents that can harm development before birth.

HGD 0191 Teratogens

A teratogen is any disease, drug or environmental agent that can harm a developing embryo or fetus, in body, later health or behaviour: radiation, alcohol, the rubella virus and certain medications among them. A teratogen's harm may not show at birth; some effects appear later, in health or behaviour.

The payoff: what decides whether harm occurs.

HGD 0192 How Teratogens Work

Whether a teratogen causes harm depends on several principles, and they answer this part's question. Timing matters most: a structure is most vulnerable while it is forming, so the embryonic period is the riskiest, and the nervous system stays susceptible throughout. Dose and duration matter. Genetic susceptibility and the overall prenatal environment matter. Fathers' exposures count too, because toxins, drugs and smoking affect sperm. And the environment after birth moderates long-term effects. The misconception is that a teratogen either harms or it does not. Outcomes are probabilities, which is why two exposed pregnancies can end very differently.

Infections that act as teratogens.

HGD 0193 Infectious Teratogens

Infections can act as teratogens: rubella in the first three months, cytomegalovirus, syphilis, toxoplasmosis, the Zika virus and HIV. Zika can cause microcephaly, an unusually small head, in exposed infants. Antiviral treatment cuts HIV transmission to a newborn below five percent. Listeria, a bacterium found in some foods, is another risk.

Drugs and medications.

HGD 0194 Drugs and Medications

Drugs that act as teratogens include some prescribed medications and psychoactive substances such as cocaine, opioids, nicotine, cannabis and alcohol. Thalidomide, once prescribed in pregnancy, left children with very short or missing limbs. Its harm came when it was taken early, during limb formation, a vivid case of the timing principle.

The most common drug teratogen.

HGD 0195 Fetal Alcohol Spectrum Disorder

Fetal alcohol spectrum disorder names the range of conditions in a child caused by alcohol exposure before birth. Alcohol stands beside nicotine, opioids and cannabis among the psychoactive teratogens. Because the nervous system stays susceptible throughout pregnancy, alcohol's risk is not confined to the early weeks of organ formation.

Hazards in the environment.

HGD 0196 Environmental Teratogens

Environmental teratogens include radiation, lead, mercury and PCBs, industrial chemicals, which can reach the embryo or fetus through the mother; lead from old pipes or paint becomes a hazard long before a baby is born. Fathers' exposure matters here too: toxins and smoking affect sperm, and secondhand smoke lowers birth weight.

Look at what you can now do. You can explain differential susceptibility: the short-allele adolescents did worse with poor family support and better with strong support, so sensitivity cuts both ways. You can put the three periods in order, germinal to about two weeks, embryonic from week three to week eight, and fetal for the last seven months, and say that the periods do not line up with the trimesters. You can tell a miscarriage, a loss in the first twenty weeks, from a stillbirth, at or after the twentieth week in the United States. You can explain what decides whether a teratogen causes harm: timing most, then dose and duration, genes, the father's exposures, and the environment after birth. And you can name teratogens of each kind: rubella and Zika, thalidomide and alcohol, lead and mercury. Part three turns to stress and maternal age.