There are two types of sex cells or gametes involved in reproduction: the male gametes or sperm and female gametes or ova. The male gametes are produced in the testes in a process called spermatogenesis, which begin at about 12 years of age. The female gametes or ova, which are stored in the ovaries, are present at birth but are immature. Each ovary contains about 250,000 (Rome 1998) but only about 400 of these will become mature eggs (Mackon and Fauser 2000). Beginning at puberty, one ovum ripens and is released about every 28 days, a process called oogenesis.
After the ovum or egg ripens and is released from the ovary, it is drawn into the fallopian tube and in 3 to 4 days, reaches the uterus. It is typically fertilized in the fallopian tube and continues its journey to the uterus. At ejaculation, millions of sperm are released into the vagina, but only a few reach the egg and typically, only one fertilizes the egg. Once a single sperm has entered the wall of the egg, the wall becomes hard and prevents other sperm from entering. After the sperm has entered the egg, the tail of the sperm breaks off and the head of the sperm, containing the genetic information from the father, unites with the nucleus of the egg. As a result, a new cell is formed. This cell, containing the combined genetic information from both parents, is referred to as a zygote.
Chromosomes contain genetic information from each parent. While other normal human cells have 46 chromosomes (or 23 pair), gametes contain 23 chromosomes. In a process called meiosis, segments of the chromosomes from each parent form pairs and genetic segments are exchanged as determined by chance. Because of the unpredictability of this exchange the likelihood of having offspring that are genetically identical (and not twins) is one in trillions (Gould and Keeton, 1997).
Determining the Sex of the Child
Twenty-two of those chromosomes from each parent are similar in length to a corresponding chromosome from the other parent. However, the remaining chromosome looks like an X or a Y. Half of the male’s sperm contain a Y chromosome and half contain an X. All of the ova contain two X chromosomes. If the child receives the combination of XY, the child will be genetically male. If it receives the XX combination, the child will be genetically female.
Many potential parents have a clear preference for having a boy or a girl and would like to determine the sex of the child.
Through the years, a number of tips have been offered for the potential parents to maximize their chances for having either a son or daughter as they prefer. For example, it has been suggested that sperm, which carry a Y chromosome are more fragile than those carrying an X. So, if a couple desires a male child, they can take measures to maximize the chance that the Y sperm reaches the egg. This involves having intercourse 48 hours after ovulation, which helps the Y sperm have a shorter journey to reach the egg, douching to create a more alkaline environment in the vagina, and having the female reach orgasm first so that sperm are not pushed out of the vagina during orgasm. (27)
Now we turn our attention to prenatal development, which is divided into three periods: the germinal period, the embryonic period, and the fetal period. Here is an overview of some of the changes that take place during each period. (28)
The germinal period (about 14 days in length) lasts from conception to implantation of the zygote (fertilized egg) in the lining of the uterus. During this time, the organism begins cell division and growth. After the fourth doubling, differentiation of the cells begins to occur as well. It’s estimated that about 60 percent of natural conceptions fail to implant in the uterus. The rate is higher for in vitro conceptions. (28)
The Embryonic Period
This period begins once the organism is implanted in the uterine wall. It lasts from the third through the eighth week after conception. During this period, cells continue to differentiate and at 22 days after conception the neural tube forms which will become the brain and spinal column. By day 24 – 28, the heart is beating. Growth during prenatal development occurs in two major directions:
- From head to tail (cephalocaudal development)
- From the midline outward (proximodistal development)
This means that those structures nearest the head develop before those nearest the feet and those structures nearest the torso develop before those away from the center of the body (such as hands and fingers). In the early stages of the embryonic period, gills and a tail are apparent. But by the sixth week, the organism looks human. About 20 percent of embryos fail during the embryonic period, usually due to gross chromosomal abnormalities. It is during this stage that the major structures of the body are taking form making the embryonic period the time when the organism is most vulnerable to the greatest amount of damage if exposed to harmful substances. Potential mothers are not often aware of the risks they introduce to the developing child during this time. The period ends with organogenesis, which means that every single organ is present in the baby. The organs only need time to grow and mature, with some organs not maturing until early adulthood! The embryo is approximately 1 inch in length and weighs about 4 grams at the end of this period. The embryo can move and respond to touch at this time.
The Fetal Period
From the ninth week until birth, the organism is referred to as a fetus. During this stage, the major structures are continuing to develop. By the 12th week, the external genitalia can be identified during an ultrasound. In the following weeks, the fetus will develop hair, nails, and teeth. The fetus will move away from painful or strange stimuli in the womb. At the end of the 12th week, the fetus is about 3 inches long and weighs about 28 grams.
During the 4-6th months, the eyes become more sensitive to light and hearing develops. Respiratory system continues to develop. Cycles of sleep and wakefulness are present. The first chance of survival outside the womb, known as the age of viability, is reached at about 22 and 24 weeks (Moore & Persaud, 1998). Many practitioners hesitate to resuscitation before 24 weeks. The majority of the neurons in the brain have developed by 24.
Between the 7th and 9th months the fetus is primarily preparing for birth. It is exercising its muscles, its lungs begin to expand and contract. It is developing fat layers under the skin. The fetus gains about 5 pounds and 7 inches during this last trimester of pregnancy, which includes a layer of fat gained during the 8th month. This layer of fat serves as insulation and helps the baby regulate body temperature after birth. (28)
Dangers during Prenatal Development
Good prenatal care is essential. The developing child is most at risk for some of the most severe problems during the first three months of development. Unfortunately, this is a time at which most mothers are unaware that they are pregnant. Today, we know many of the factors that can jeopardize the health of the developing child. The study of factors that contribute to birth defects is called teratology. Teratogens are factors that can contribute to birth defects, which include some maternal diseases, pollutants, drugs and alcohol.
There are several considerations in determining the type and amount of damage that might result from exposure to a particular teratogen (Berger, 2004). Factors influencing prenatal risks:
The Timing of the Exposure
Structures in the body are vulnerable to the most severe damage when they are forming. If a substance is introduced during a particular structure’s critical period (time of development), the damage to that structure may be greater. For example, the ears and arms reach their critical periods at about 6 weeks after conception. If a mother exposes the embryo to certain substances during this period, the arms and ears may be malformed.
The amount of exposure:
Some substances are not harmful unless the amounts reach a certain level. The critical level depends in part on the size and metabolism of the mother.
Genetic make-up also plays a role on the impact a particular teratogen might have on the child. This is suggested by fraternal twin studies who are exposed to the same prenatal environment, yet do not experience the same teratogenic effects. The genetic make-up of the mother can also have an effect; some mothers may be more resistant to teratogenic effects than others.
Being male or female:
Males are more likely to experience damage due to teratogens than are females. It is believed that the Y chromosome, which contains fewer genes than the X, may have an impact. (29)
A Look at Some Teratogens
One of the most commonly used teratogens is alcohol and because half of all pregnancies in the United States are unplanned, it is recommended that women of child-bearing age take great caution against drinking alcohol when not using birth control or when pregnant (Surgeon General’s Advisory on Alcohol Use During Pregnancy, 2005).
Alcohol consumption, particularly during the second month of prenatal development but at any point during pregnancy, may lead to neurocognitive and behavioral difficulties that can last a lifetime. Binge drinking (5 or more on a single occasion) or 7 or more drinks during a single week place a child at risk. In extreme cases, alcohol consumption can lead to fetal death but more frequently it can result in fetal alcohol spectrum disorders (FASD) (this terminology is now used when looking at the effects of exposure and replaces the term fetal alcohol syndrome. It is preferred because it recognizes that symptoms occur on a spectrum and that all individuals do not have the same characteristics.)
Children with FASD share certain physical features such as flattened noses, small eyes, and small heads, intellectual developmental delay, and behavioral problems. Those with FASD are more at risk for lifelong problems such as criminal behavior, psychiatric problems, and unemployment (CDC, 2006). The terms alcohol-related neurological disorder (ARND) and alcohol-related birth defects (ARBD) have replaced the term Fetal Alcohol Effects to refer to those with less extreme symptoms of FASD. ARBD include kidney, bone and heart problems.
Tobacco is the second most widely used teratogen and the number of adolescent females who smoke is increasing. In fact, among adolescents, females are just as likely to smoke as are males. Tobacco use during pregnancy has been associated with low birth weight, placenta previa, preterm delivery, fetal growth restriction and sudden infant death syndrome (Center for Disease Control, 2004).
Illicit drugs as well as prescribed medications can have serious teratogenic effects. It is difficult to completely determine the effects of a particular illicit drug on a developing child because most mothers, who use, use more than one substance. However, several problems seem clear. The use of cocaine is connected with low birth weight, stillbirths and spontaneous abortion. Heavy marijuana use is associated with brain damage and mothers addicted to heroin often pass that addiction to their child. And many medications do not include adequate information on risks posed if taken during pregnancy (Center for Disease Control, 2004).
Some environmental pollutants of major concern include lead poisoning, which is connected with low birth weight and slowed neurological development. Children who live in older housing in which lead based paints have been used have been known to eat peeling paint chips thus being exposed to lead. The chemicals in certain herbicides are also potentially damaging. Radiation is another environmental hazard. If a mother is exposed to radiation, particularly during the first 3 months of pregnancy, the child may suffer some congenital deformities. There is also an increased risk of miscarriage and stillbirth. Mercury leads to physical deformities and mental retardation (Dietrich, 1999).
Human Immunodeficiency Virus (HIV)
One of the most potentially devastating teratogens is HIV. In the United States, the fastest growing group of people with Acquired Immunodeficiency Syndrome (AIDS) is women; globally half of all people infected with HIV are women (UNAIDS, 2005). It is estimated that between 630,000 to 820,000 children were newly infected with HIV worldwide in 2005. Most of this infection is from mother-to-child through the placenta or birth canal (Newell, 2005). There are some measures that can be taken to lower the chance the child will contract the disease (such as the use of antiretroviral drugs from 14 weeks after conception until birth, avoiding breastfeeding, and delivering the child by c-section). Many women do not know they are HIV positive during pregnancy. Still others cannot afford the costly drugs used for treating AIDS. The transmission rate of HIV from mother to child has been reduced in the United States to between 100-200 infants annually.
German measles (or rubella) have been associated with a number of maladies. If the mother contracts the disease during the first three months of pregnancy, damage can occur in the eyes, ears, heart or brain of the unborn child. Deafness is almost certain if the mother has German measles before the 11th week of prenatal development and can also cause brain damage. Gonorrhea, syphilis, and Chlamydia are sexually transmitted infections that can be passed to the fetus by an infected mother; mothers should be tested as early as possible to minimize the risk of spreading these infections (Center for Disease Control, 2006). (29)
Common Health Conditions during Pregnancy
Common health conditions during pregnancy include:
- Urinary Tract Infections
- Preeclampsia and Eclampsia
- Mental health conditions
- Hypertension (High Blood Pressure)
- Gestational Diabetes Mellitus (GDM)
- Obesity and Weight gain
- Hyperemesis Gravidarum
Each of these conditions will be discussed next.
Anemia is having lower than the normal number of healthy red blood cells. Treating the underlying cause of the anemia will help restore the number of healthy red blood cells. Women with pregnancy related anemia may feel tired and weak. This can be helped by taking iron and folic acid supplements.
Urinary Tract Infections (UTI)
A UTI is a bacterial infection in the urinary tract. UTI symptoms include:
- Pain or burning when you use the bathroom
- Fever, tiredness, or shakiness.
- An urge to use the bathroom often.
- Pressure in your lower belly.
- Urine that smells bad or looks cloudy or reddish.
- Nausea or back pain.
Treatment with antibiotics to kill the infection will make it better, often in one or two days. (30)
Preeclampsia and Eclampsia
Preeclampsia (pree-i-KLAMP-see-uh) and eclampsia (ih-KLAMP-see-uh) are pregnancy-related high blood pressure disorders. In preeclampsia, the mother’s high blood pressure reduces the blood supply to the fetus, which may get less oxygen and fewer nutrients. (31)
Preeclampsia is similar to gestational hypertension, because it also describes high blood pressure at or after 20 weeks of pregnancy in a woman whose blood pressure was normal before pregnancy. But preeclampsia can also include blood pressure at or greater than 140/90 mmHg, increased swelling, and protein in the urine. The condition can be serious and is a leading cause of preterm birth (before 37 weeks of pregnancy). If it is severe enough to affect brain function, causing seizures or coma, it is called eclampsia. (32)
Mental Health Conditions
Some women experience depression during or after pregnancy. Symptoms of depression are:
- A low or sad mood
- Loss of interest in fun activities
- Changes in appetite, sleep, and energy
- Problems thinking, concentrating, and making decisions
- Thoughts that life is not worth living
When many of these symptoms occur together and last for more than a week or two at a time, this is probably depression. Depression that persists during pregnancy can make it hard for a woman to care for herself and her unborn baby. Having depression before pregnancy also is a risk factor for postpartum depression. Getting treatment is important for both mother and baby. (30)
Chronic poorly-controlled high blood pressure before and during pregnancy puts a pregnant woman and her baby at risk for problems. It is associated with an increased risk for maternal complications such as preeclampsia placental abruption (when the placenta separates from the wall of the uterus), and gestational diabetes. These women also face a higher risk for poor birth outcomes such as preterm delivery, having an infant small for his/her gestational age, and infant death. Getting treatment for high blood pressure is important before, during, and after pregnancy.
Gestational Diabetes Mellitus (GDM)
GDM is diagnosed during pregnancy and can lead to pregnancy complications. GDM is when the body cannot effectively process sugars and starches (carbohydrates), leading to high sugar levels in the blood stream. Most women with GDM can control their blood sugar levels by a following a healthy meal plan from their health care provider and getting regular physical activity. Some women also need insulin to keep blood sugar levels under control. Doing so is important because poorly controlled diabetes increases the risk of:
- Early delivery
- Cesarean birth
- Having a baby born with low blood sugar, breathing problems, and jaundice
Although GDM usually resolves after pregnancy, women who had GDM have a higher risk of developing diabetes in the future.
Obesity and Weight Gain
Recent studies suggest that the heavier a woman is before she becomes pregnant, the greater her risk of pregnancy complications, including preeclampsia, GDM, stillbirth and cesarean delivery. Also, CDC Research has shown that obesity during pregnancy is associated with increased use of health care and physician services, and longer hospital stays for delivery. Overweight and obese women who lose weight before pregnancy are likely to have healthier pregnancies.
Many women have some nausea or vomiting, or “morning sickness,” particularly during the first 3 months of pregnancy. The cause of nausea and vomiting during pregnancy is believed to be rapidly rising blood levels of a hormone called HCG (human chorionic gonadotropin), which is released by the placenta. However, hyperemesis gravidarum occurs when there is severe, persistent nausea and vomiting during pregnancy—more extreme than “morning sickness.” This can lead to weight loss and dehydration and may require intensive treatment. (30)