Prenatal Development

The Role of Genes in Prenatal Development

Prenatal development is highly influenced by the inheritance, expression, and regulation of genes.

Prenatal Development

Developmental psychologists consider the process of human development as it relates to physical, cognitive, and psychosocial development. This lifespan development is organized into different stages based on age. Prenatal development is the process that occurs during the 40 weeks prior to the birth of a child, and is heavily influenced by genetics.

There are three stages of prenatal development—germinal, embryonic, and fetal. Prenatal development is also organized into trimesters: the first trimester ends with the end of the embryonic stage, the second trimester ends at week 20, and the third trimester ends at birth.

Overview of Genetic Inheritance

Chromosomes

Every person is made up of cells, each of which contains chromosomes. Chromosomes are genetic material that determines many things about a person, such as eye and hair color, biological sex, and personality traits. DNA, or deoxyribonucleic acid, is responsible for the transmission of genetic material. A mother and father’s DNA are passed on at the moment of conception.

A human being has a total of 23 pairs of chromosomes. The developing zygote gets half of its chromosomes from one parent and half from the other parent. The first 22 pairs of chromosomes are known as autosomes and determine things such as eye and hair color. The last pair, known as the sex chromosomes, determine a person's biological sex: females have two X chromosomes, while males have an X and a Y chromosome.

Gene Expression and Regulation

Gene expression is carefully regulated in every organism to allow the organism to adapt to differing conditions. The expression of genetic information in a given cell or organism is neither random nor fully pre-programmed. Genes can either be dominant or recessive, meaning they can either be expressed or hidden. Depending on the dominance of each chromosome that is inherited from each parent, the child may or may not show the inherited trait.

Gene regulation is the process by which cells differentiate. Among other things, it is the process in which a cell determines which genes it will express and when. Cell differentiation is a process by which a less specialized cell becomes a more specialized cell. For example, as a zygote develops, gene regulation changes some cells into brain cells and others into liver cells. Other cells will become the lining of the stomach, the intestines, and the sexual reproductive organs.

Mutation is when a sudden change in a segment of the DNA occurs. Some mutations of the genes can result in conditions such as Down Syndrome or Turner's Syndrome.

The Importance of Genetics in Human Development

Differences in gene expression—whether as a result of standard regulation processes or through mutation—are crucial to an individual's physical and psychological development. The exact extent to which genes, as opposed to an individual's environment, determine or influence psychological development is hotly debated; this controversy is known as the "nature-vs.-nurture debate." However, an individual's genetic makeup at the very least serves as a crucial baseline (which may then be mediated by the environment) for such characteristics as the ability to begin learning spoken language, such personality traits as a tendency toward aggressive versus submissive behavior, and risk levels for such diseases as alcoholism and addiction.

Before birth, a fetus has of course had limited opportunity to be shaped by its environment, beyond factors such as the mother's diet, substance use, and anxiety level. For this reason, genetics play a particularly important role in prenatal development.

Prenatal Brain Development

Prenatal development is the process of rapid change and growth that occurs in the 40 weeks prior to the birth of a child.

Prenatal development is the process that occurs during the 40 weeks prior to the birth of a child. There are three stages of prenatal development: germinal, embryonic, and fetal. Prenatal development is also organized into three equal trimesters, which do not correspond with the three stages. The first trimester ends with the end of the embryonic stage, the second trimester ends at week 20, and the third trimester ends at birth.

Germinal Stage

The germinal stage is the stage of development that occurs from conception until 2 weeks (implantation). Conception occurs when a sperm fertilizes an egg and forms a zygote. A zygote begins as a one-cell structure that is created when a sperm and egg merge. At the moment of conception, the mother's and father’s DNA are passed on to; the genetic makeup and sex of the future fetus are set at this point. During the first week after conception, the zygote rapidly divides and multiplies, going from a one-cell structure to two cells, then four cells, then eight cells, and so on. This process of cell division is called mitosis. Mitosis is a fragile process, and fewer than one-half of all zygotes survive beyond the first two weeks (Hall, 2004). After 5 days of mitosis there are 100 cells, and after 9 months there are billions of cells. As the cells divide, they become more specialized, forming different organs and body parts. During the germinal stage, the cells necessary for the placenta, umbilical cord, and amniotic fluid will differentiate to form the embryo. The mass of cells has yet to attach itself to the lining of the uterus; once this attachment occurs, the next stage begins.

Embryonic Stage

The embryonic stage lasts from implantation (2 weeks) until week 8 of pregnancy. After the zygote divides for about 7–10 days and has 150 cells, it travels down the fallopian tubes and implants itself in the lining of the uterus. Upon implantation, this multi-cellular organism is called an embryo. Now blood vessels grow, forming the placenta. The placenta is a structure connected to the uterus that provides nourishment and oxygen from the woman's body to the developing embryo through the umbilical cord.

During the first week of the embryonic period, the embryonic disk separates into three layers: the ectoderm, mesoderm, and endoderm. The ectoderm is the layer that will become the nervous system and outer skin layers; the mesoderm will become the circulatory system, skeleton, muscles, reproductive system, and inner layer of skin; and the endoderm will become the respiratory system and part of the digestive system, as well as the urinary tract.

The first part of the embryo to develop is the neural tube, which will become the spinal cord and brain. As the nervous system starts to develop, the tiny heart starts to pump blood, and other parts of the body—such as the digestive tract and backbone—begin to emerge. In the second half of this period, growth is very rapid. The eyes, ears, nose, and jaw develop; the heart develops chambers; and the intestines grow.

Fetal Stage

The remainder of prenatal development occurs during the fetal stage, which lasts from week 9 until birth (usually between 38 and 40 weeks). When the organism is about nine weeks old, the embryo is called a fetus. At this stage, the fetus is about the size of a kidney bean and begins to take on the recognizable form of a human being. Between 9 and 12 weeks, reflexes begin to appear and the arm and legs start to move (those first movements won't be felt for a few weeks, however). During this same time, the sex organs begin to differentiate. At about 16 weeks, the fetus is approximately 4.5 inches long. Fingers and toes are fully developed, and fingerprints are visible. By the time the fetus reaches the sixth month of development (24 weeks), it weighs up to 1.4 pounds. Hearing has developed, so the fetus can respond to sounds. The internal organs, including the lungs, heart, stomach, and intestines, have formed enough that a fetus born prematurely at this point has a chance to survive outside of the womb.

Throughout the fetal stage the brain continues to grow and develop, nearly doubling in size from weeks 16 to 28. Brain growth during this period allows the fetus to develop new behaviors. The cerebral cortex grows larger, and the fetus spends more hours awake. The fetus moves with more coordination, indicating more neural connections within the brain. The nervous system is controlling more bodily functions, and even personality begins to emerge in utero. By 28 weeks, thalamic brain connections form, which mediate sensory input. The fetus can distinguish between voices, and can remember songs and certain sounds after birth. The fetus becomes sensitive to light as well; in fact, if a doctor shines a light on the womb, the baby will attempt to shield his or her eyes. Growth begins to slow around 30 to 32 weeks, but small changes continue until birth.

Around 36 weeks, the fetus is almost ready for birth. It weighs about 6 pounds and is about 18.5 inches long, and by week 37 all of the fetus’s organ systems are developed enough that it could survive outside the uterus without many of the risks associated with premature birth. The fetus continues to gain weight and grow in length until approximately 40 weeks. By then, the fetus has very little room to move around and birth becomes imminent.

Environmental Impacts on Prenatal Development

Environmental factors, such as exposure to teratogens, can have a range of impacts on the developing fetus.

Prenatal development is the process that occurs during the 40 weeks prior to the birth of a child. During each prenatal stage, environmental factors affect the development of the fetus. The developing fetus is completely dependent on the mother for life, and it is important that the mother receives prenatal care, which is medical care during pregnancy that monitors the health of both the mother and the fetus. According to the National Institutes of Health ([NIH], 2013), routine prenatal care can reduce the risk of complications to the mother and fetus during pregnancy.

When the zygote attaches to the wall of the uterus, the placenta is formed. The placenta provides nourishment and oxygen to the fetus. Most everything the mother ingests, including food, liquid, and even medication, travels through the placenta to the fetus—hence the common phrase that a mother “eats for two.” Anything the mother is exposed to in the environment affects the fetus; if the mother is exposed to something harmful, the child can show life-long effects.

Teratogens

A teratogen is any environmental substance or agent—biological, chemical, or physical—that can have a detrimental effect on a developing fetus. Exposure to teratogens during the prenatal stage can significantly raise the risk of birth defects. Several factors influence the amount of damage a teratogen can have, including dose or level of exposure, heredity, age of the teratogen, and any other negative influences (for example, several teratogens or a teratogen combined with poor health). There are several known teratogens that expectant mothers are advised to avoid during pregnancy, including alcohol, prescription and/or illegal drugs, and tobacco.

Alcohol

Alcohol and most drugs cross the placenta and affect the fetus. Alcohol use during pregnancy has been found to be the leading preventable cause of mental disabilities in children in the United States (Maier & West, 2001). Excessive maternal drinking while pregnant can cause fetal alcohol spectrum disorders (FASD) with life-long consequences for the child, ranging in severity from minor to major. It is unknown how much alcohol is necessary to cause damage, and so doctors typically recommend that alcohol should be completely avoided during pregnancy. Physically, children with FASD may have a small head size and abnormal facial features. Cognitively, these children may have poor judgment, poor impulse control, higher rates of ADHD and learning issues, and lower IQ scores. These developmental problems and delays persist into adulthood (Streissguth et al., 2004). Based on studies conducted on animals, it also has been suggested that a mother’s alcohol consumption during pregnancy may predispose her child to like alcohol (Youngentob et al., 2007).

Each organ of the fetus develops during a specific period in the pregnancy, called the critical or sensitive period. Research into FASD has demonstrated that the time during which a developing fetus is exposed to alcohol can dramatically affect the appearance of facial characteristics associated with FASD. Specifically, this research suggests that alcohol exposure that is limited to day 19 or 20 of gestation can lead to significant facial abnormalities in the offspring of primates (Ashley, Magnuson, Omnell, & Clarren, 1999). Given regions of the brain also show sensitive periods during which they are most susceptible to the teratogenic effects of alcohol (Tran & Kelly, 2003).

Prescription and/or Illegal Drugs

Use of any type of drug—whether illegal, prescription, or over-the-counter—can be dangerous during pregnancy. Illegal drugs such as heroine, cocaine, and methamphetamine can cause a myriad of problems for the developing fetus: babies can be born addicted to certain drugs and are also more likely to be born prematurely, have low birth weight, and experience other physical defects. Many end up with attention and behavioral problems as well.

Prescription drugs taken during pregnancy such as streptomycin, tetracycline, some antidepressants, progestin, synthetic estrogen, Accutane, thalidomide, and diethylstilbestrol (known as DES)—as well as over-the-counter drugs such as diet pills—can also result in teratogenic outcomes for the developing fetus. Thalidomide causes bodily deformities as well as damage to internal organs. DES-exposed fetuses have been shown to have higher rates of cancer and infertility as adults. Additionally, high doses of aspirin are known to lead to maternal and fetal bleeding, although low-dose aspirin is usually not harmful. The classification of a drug (as A, B, C, D, or X) allows a mother to make determinations about using drugs during pregnancy: for example, class A drugs are deemed always safe, whereas class X drugs have proven to be damaging to the fetus.

Smoking

Smoking tobacco is also considered a teratogen because nicotine travels through the placenta to the fetus. When the mother smokes, the developing baby experiences a reduction in blood oxygen levels. According to the Centers for Disease Control and Prevention (2013), smoking while pregnant can result in premature birth, low-birth-weight infants, stillbirth, and sudden infant death syndrome (SIDS)—the sudden and unexplained death of a child less than one year of age. Other issues that can be caused by prenatal exposure to smoking are inattentiveness, muscle tension, and colic (a form of pain which starts and stops abruptly and occurs due to muscular contractions in the body). The more a mother smokes or is exposed to second-hand smoke, the greater the risk; however, quitting (even after smoking during pregnancy) greatly reduces the risks of these problems.

Other Teratogens

Other teratogens that affect prenatal development include radiation, pollution, and infectious disease. Radiation increases the risk of childhood cancer, as well as emotional and behavioral disorders; because of this, it is recommended that pregnant women avoid x-rays unless absolutely necessary. Pollution, such as exposure to mercury or PCBs, can cause physical deformities, abnormal speech, and difficulty with coordination. Maternal infections such as viruses or parasites can also cause brain damage to the fetus, or even death.

Maternal Stress and Depression

Any form of prenatal stress felt by the mother can have negative effects on various aspects of fetal development, and can cause harm to both mother and child. When a mother is under stress, physiological changes occur in the body that could harm the developing fetus. Additionally, a stressed mother is more likely to engage in behaviors that could negatively affect the fetus, such as smoking, drug use, and alcohol abuse. Prenatal depression is often caused by the stress and worry that pregnancy can bring, only at a more severe level. Other factors that can put a person at risk for prenatal depression include unplanned pregnancy, difficulty becoming pregnant, history of abuse, and economic or family problems.

The use of antidepressants in pregnancy, mentioned above, has been associated with a variety of risks for the fetus with varying degrees of proof of causation. While some studies clearly show the adverse outcomes of prenatal antidepressant exposure, others are less clear—and complications arise because depression itself is independently associated with negative pregnancy outcomes. Determining the extent to which adverse outcomes are caused by antidepressant use or by depression—or a combination of both—is difficult to measure; it is also important to factor in the negative consequences of a mother going off prescription antidepressants during pregnancy, which may adversely effect her health in other ways.