Pregnancy is the period of time between conception (kun-SEP-shun) and birth. A full-term pregnancy lasts nine months and usually does not involve major health problems. Sometimes, however, complications develop that jeopardize the health of the mother and/or baby.

What Is Pregnancy?

Pregnancy usually lasts 40 weeks in humans, beginning from the first day of the woman's last menstrual period, and is divided into three trimesters. The illustration depicts the position of the developing fetus during each trimester.

Pregnancy usually lasts 40 weeks in humans, beginning from the first day of the woman's last menstrual period, and is divided into three trimesters. The illustration depicts the position of the developing fetus during each trimester.
Illustration by Electronic Illustrators Group. © 2016 Cengage Learning®.
Fertilization and implantation

Eggs are formed in the ovaries, a part of the female reproductive tract. Each month, one of the two ovaries forms egg follicles, fluid-filled cysts that have the potential to grow into mature eggs. Only one follicle actually creates and releases a mature egg each month. In releasing the egg, the follicle ruptures. The cells that made up the follicle then begin to secrete hormones * that are needed to maintain a pregnancy. The egg released from the ovary travels down the fallopian tube toward the uterus * . On this journey, if sperm * meets and fertilizes the egg in time to implant in the uterus, pregnancy has the potential to occur. Sperm and egg actually meet in the fallopian tube where a sperm fertilizes the egg. One sperm cell is all that is required to fertilize an egg.

Normal fertilization takes place over approximately 24 hours. During this time the outside of the egg changes to ensure that other sperm cells will not penetrate. Once the egg is fertilized, all of the genetic material necessary for a new life is present, and the gender is determined. The female can only contribute an X chromosome * (females are XX). Males may contribute an X or Y chromosome (males are XY). If the sperm that fertilizes the egg is carrying an X chromosome, the result will be female. If the sperm carries a Y chromosome, the result will be male. The fertilized egg is referred to as an embryo * * to prevent the entry of more sperm.

Human chorionic gonadotropin (hCG) is a hormone produced by the placental tissue growing near the embryo used in pregnancy tests. This hormone is present in the maternal blood about one week after conception, but not present in the urine for a home pregnancy test until three to four weeks into the pregnancy. Throughout pregnancy, distinct and intricately synchronized hormonal changes are necessary for a fetus to make it to full term. Progesterone is the hormone necessary for a successful pregnancy to occur, and it also prevents menstrual cycles * during pregnancy.

First trimester

The first trimester lasts about 12 weeks. Week three to week eight are referred to as the embryonic stage of pregnancy. In this period, the internal organs such as the heart are forming, and the embryo is especially vulnerable to damaging outside factors such as alcohol, bacterial infections, viruses, and toxins that would not significantly harm an adult body. The nervous system, including the brain, forms around the third week of pregnancy. Proper formation of the nervous system requires an adequate amount of vitamins, including folic acid, one of the B vitamins. Without proper nutrition, an embryo may become malformed or lost. Around the fifth week of pregnancy the heart begins to beat. After the eighth week, the embryo is referred to as a fetus * .

Second trimester

The second trimester lasts from week 13 to week 27. During this stage the fetus rapidly increases in size and length. Organs continue to grow and move into their final physical locations. Skin is formed, blood creation begins, movement occurs, and the fetus begins to hear sounds. The lungs are beginning to produce a substance known as surfactant, which allows the air sacs in the lungs to inflate properly and not collapse when they deflate. Without the surfactant, a baby cannot breathe outside of the body of the mother. This is one of the reasons why a baby born too early may not survive.


A cesarean section (si-ZAR-ee-an SEK-shun) is the method of delivering a child by surgically opening the abdomen and uterus. It is performed when delivery through the vagina presents risk to the mother or child.

The name of the procedure comes from the traditional story that the Roman ruler Julius Caesar (100–44 BCE) was delivered by such surgical means. More likely is the explanation that Cesarean law forbade the burial of a deceased mother before the baby was delivered.

Written accounts of the rescuing of an infant from its dead mother were recorded as early as 500 BCE. Cesarean sections were known to have been practiced by the ancient Romans, Indians, and Jews in the Roman era.

General weight-gain recommendations for women who are

Third trimester

The third trimester lasts from about week 28 to the time of birth, usually around week 40. Week 40 is used to calculate the due date, but a full-term pregnancy may be anywhere between 38 and 42 weeks. During the third trimester the fetus gains significant weight and the eyes open. The fetus can detect light and begins making antibodies as part of the immune system. After 38 to 40 weeks, the baby is delivered through the vagina * or by cesarean section * .

What Are the Normal Discomforts of Pregnancy?

As a woman gains weight and her body changes to accommodate a growing fetus, she may experience some of the following signs or symptoms:

Symptoms vary from woman to woman, and even between pregnancies for an individual woman.

What Are the Common Complications of Pregnancy?

How Common Are the Common Pregnancy Complications?

The approximate percentages of pregnant women affected by the common complications of pregnancy are as follows:

  • Ectopic pregnancy: 2 percent
  • Gestational diabetes: 1 to 10 percent
  • Miscarriage: as many as 40 percent of pregnancies end in miscarriage, often before a woman even knows she is pregnant. Of confirmed pregnancies, about 10 percent end in miscarriage.
  • Morning sickness: 50 percent. Severe nausea and vomiting that require medical care occur in less than 0.5 percent of pregnancies.
  • Placenta previa: 1 percent
  • Premature birth: 7 to 12 percent of deliveries. About one-third of these are caused by preterm labor.
  • Preeclampsia/toxemia: 5 to 10 percent
Hyperemesis gravidarum (morning sickness)

Hyperemesis gravidarum (morning sickness) is the name for the nausea and vomiting some women experience during pregnancy. But it is misnamed in some cases. Some women have it in the morning, others at noon, and still others at night. Some women feel queasy only occasionally, whereas others feel sick all day long. Overall, about half of all pregnant women experience some degree of morning sickness, but it usually subsides on its own after about the third month of pregnancy. Only rarely does it require medical attention. The cause of morning sickness is unclear, but it is probably related to the hormonal and other chemical shifts that occur in early pregnancy.


Miscarriage (MIS-kare-ij) is also called spontaneous abortion; it occurs when a pregnancy suddenly ends on its own. Bleeding, cramping, and abdominal pain often signal a miscarriage. Most miscarriages occur before the 14th week of pregnancy, which is why many women wait to tell their family and friends about a pregnancy until they have passed the three-month mark. Occasionally, a woman will have a “late” miscarriage, which means that it occurs during the second trimester. After the 20th week, the unexpected end of a pregnancy is called stillbirth if the baby is born dead and premature birth if the baby is alive but born before the 37th week.

As many as 40 percent of pregnancies end in miscarriage, although most of these occur so early that a woman may not even realize that she is pregnant. Early miscarriages often occur when the body naturally rejects an embryo that is not developing properly. Later miscarriages are much less common. Reasons for late miscarriages include a placenta * that is improperly attached to the uterus, the placenta separating from the wall of the uterus for some reason, and other causes.

Gestational diabetes

Gestational diabetes is a type of diabetes that occurs when a woman does not produce enough insulin to handle the increased blood sugar that accompanies pregnancy. Any woman can develop this common problem, but women who are older, are overweight, and who have relatives with diabetes are at higher risk. A special diet often can control the problem without medication. Untreated diabetes during pregnancy increases the risk of certain birth defects. Such infants often have abnormally high birth weights and are prone to developing low blood sugar in the hours after birth. Most cases of gestational diabetes are temporary and disappear after the baby is born.

Ectopic pregnancy * . Cramps, nausea, dizziness, tenderness in the lower abdomen * , and light vaginal bleeding often accompany ectopic pregnancies. Early detection and treatment of an ectopic pregnancy are essential. If left untreated, the condition can lead to rupture of the fallopian tube and then to massive internal bleeding and shock, thus becoming life threatening to the mother.

An ultrasound * can be used to examine the abdomen and confirm the diagnosis of an ectopic pregnancy. An ultrasound sends sound waves into the body that bounce off internal structures. A computer converts the returning sound waves into an image of the internal structures. Ectopic pregnancies usually are removed surgically.

Incompetent cervix

An incompetent cervix is the cause of about 25 percent of late miscarriages. The cervix is the muscular opening of the uterus into the vagina (vah-JY-nah). An incompetent cervix opens too early due to the pressure exerted by the growing fetus. An incompetent cervix can be caused by many factors, including a genetic tendency for it, stretching or tearing of the cervix during previous deliveries, and carrying multiple fetuses. An incompetent cervix can be treated by stitching the cervix closed during the second trimester or by bed rest for the last several months of pregnancy.


The exact cause of preeclampsia/eclampsia is unknown and likely has multiple factors. Eclampsia is defined as preeclampsia plus convulsions * . The only known treatment for advanced preeclampsia (in which blood pressure cannot be controlled with medications) or eclampsia is delivery of the baby. Most cases of preeclampsia are characterized by swelling of the face, hands, and ankles; too-rapid weight gain; headaches; and protein in the urine. When left untreated, preeclampsia can progress to eclampsia, which can cause nausea, vomiting, blurred vision, convulsions, and coma * .

Preeclampsia most often affects young women during the last months of their first pregnancy, and the cause is unknown. Often, treatment involves hospitalization until blood pressure returns to normal, followed by limited activity and sometimes bed rest at home.

Placenta previa

Placenta previa (PREE-vee-uh) is a condition in which the placenta is lying low in the uterus. It can be dangerous if the placenta actually covers the cervix during labor and delivery. During birth the baby still requires the blood, oxygen, and nutrients provided by the placenta, and so the placenta should remain attached to the uterus until after the baby is born, and then be expelled. Placenta previa can lead to premature labor, and women with this problem sometimes must limit their activity or stay in bed until the baby is born. Doctors can monitor the position of the placenta using ultrasound. When it is time for the baby to be born, doctors perform a cesarean section if the placenta is still covering or very close to the cervix.

Preterm labor and premature birth

More babies are born past their expected due date than before it, but in the United States, one in eight babies is born prematurely. A premature birth means delivery before the 37th week of pregnancy. About one-third of premature babies are born early because the mother goes into labor too soon (the other cases occur because the amniotic sac * ruptures prematurely or because a health problem with the mother or baby requires early delivery).

Among the many risk factors for preterm labor are smoking, alcohol use, drug abuse, vitamin deficiencies, a job that requires standing for long periods, infections such as German measles, placenta previa or other physical causes, and poor nutrition.

Preterm labor that results in a premature birth poses serious health problems for the baby, who has not finished developing inside the uterus.

What Are the Risk Factors for Pregnancy Complications?

Older women (over 35 years) have a higher probability than younger women of experiencing high blood pressure, diabetes, and cardiovascular disease while pregnant, but these conditions are controllable with good medical care. Older women also are more prone to miscarriage, preterm labor, and postpartum (after birth) bleeding, and they have an increased risk of having a child with birth defects.

By contrast, teen mothers are twice as likely to have premature babies and babies with low birth weight as are older mothers. Teenagers are also prone to premature labor, prolonged labor, preeclampsia, and anemia. About one-third of pregnant teens do not receive adequate medical care during pregnancy (as compared to about one-fourth of older women). Finally, while the chance of dying from pregnancy-related complications is very low overall, the rate is much higher in women younger than 15 than in women older than 15.

Can Pregnancy Complications Be Prevented?

Many complications of pregnancy develop in healthy mothers for unknown reasons. However, if a woman is in poor health before becoming pregnant, the likelihood of her having complications is higher than usual. Regular prenatal * care, or medical care during pregnancy, is very important, because it allows doctors to detect and treat problems with the mother or baby as early as possible.

Preventing Pregnancy


SOURCE: Centers for Disease Control and Prevention. “Contraception: How Effective Are Birth Control Methods?” Available online at: (accessed June 30, 2015). Table by Lumina Datamatics Ltd. © 2016 Cengage Learning®.
Contraception devices

The diaphragm and the condom are two commonly used contraception * devices. The diaphragm is a molded rubber cup with a wire embedded around its edge. A woman is measured for this form of contraception by a medical professional, who also explains how to insert and position the diaphragm across the cervix. The diaphragm provides a mechanical barrier so that sperm cannot enter the uterus through the cervix. Used with spermicidal jelly, the diaphragm is an effective method of birth control with no side effects. The condom is a thin rubber sheath worn over the erect penis that collects the sperm, preventing it from entering the woman's vagina. Another kind of condom is inserted into the vagina before intercourse and serves the same barrier function. Diaphragms are washed and reused; condoms are used only once.

Birth rates for U.S. teenagers aged 15–19, by age group, 1960–2013

Birth rates for U.S. teenagers aged 15–19, by age group, 1960–2013
SOURCE: Martin, J.A., B.E Hamilton, M.J.K Osterman, et. al., “Births: Final Data for 2013.” National Vital Statistics Reports vol 64, no 1. Hyattsville, MD: National Center for Health Statistics, 2015. Table by Lumina Datamatics Ltd. © 2016 Cengage Learning®.
Behavioral birth control

The rhythm method, also known as the fertility awareness method, involves avoiding sexual intercourse during the most fertile time in the menstrual cycle. The fertile time in a menstrual cycle is during the ovulation stage. Counting from the first day of the last menstrual cycle, ovulation generally occurs approximately 11 days later. The signs indicating the ovulation stage include increased cervical mucous and increased basal body temperature. The basal body temperature is the temperature of the body early in the morning while at rest. Basal body temperature increases slightly approximately one to two days after ovulation.

Birth control may be attempted by abstaining from sexual intercourse from day 11 and the appearance of cervical mucous until three days after the increase in basal body temperature. However, this method is often less successful because ovulation varies from person to person. Because a sperm may live in the female reproductive tract for up to seven days and the egg remains fertile for about 24 hours, a pregnancy may occur anywhere from seven days before ovulation to three days afterward. In addition, ovulation does not always occur on day 11 from the last menstrual cycle, does not always involve noticeable cervical mucous, and may have an irregular basal body temperature. Ovulation may take place anywhere from day 11 to day 21 of the cycle and at different times during each cycle. These variables cause the rhythm method of birth control to be less dependable than other methods.

See also Birth Defects: Overview • Diabetes • Ectopic Pregnancy • Infertility • Post-Partum Depression • Preeclampsia/Eclampsia • Prematurity


Books and Articles

Costa, Samantha. “When Your Pregnancy Is High Risk.” U.S. News & World Report. July 21, 2015. (accessed August 12, 2015).

Mayo Clinic Staff. Mayo Clinic Guide to a Healthy Pregnancy. Cambridge, MA: De Capo Lifelong, 2011.

Simkin, Penny, Janet Whalley, Ann Keppler, Janelle Durham, and April Bolding. Pregnancy, Childbirth, and the Newborn: The Complete Guide. Minnetonka, MN: Meadowbrook, 2016.


Centers for Disease Control and Prevention. “Pregnancy.” (accessed August 12, 2015).

Eunice Kennedy Shriver National Institute of Child Health and Human Development. “Pregnancy: Overview.” U.S. Department of Health and Human Services, National Institutes of Health. (accessed August 12, 2015).

MedlinePlus. “Pregnancy.” U.S. National Library of Medicine, National Institutes of Health. (accessed August 12, 2015).


American Pregnancy Association. 1425 Greenway Dr., Suite 440, Irving, TX 75038. Toll-free: 800-672-2296. Website: (accessed August 12, 2015).

Centers for Disease Control and Prevention. 1600 Clifton Rd., Atlanta, GA 30329-4027. Toll-free: 800-232-4636. Website: (accessed August 29, 2015).

March of Dimes. 1275 Mamaroneck Ave., White Plains, NY 10605. Telephone: 914-997-4488. Website: (accessed August 12, 2015).

Planned Parenthood Federation of America. 434 West 33rd St., New York, NY 10001. Telephone: 212-541-7800. Website: (accessed November 12, 2015).

Sidelines High-Risk Pregnancy Support. PO Box 1808, Laguna Beach, CA 92652. Toll-free: 888-447-4754. Website: (accessed August 12, 2015).

* hormones are chemical substances that are produced by various glands and sent into the bloodstream carrying messages that have certain effects on other parts of the body.

* uterus (YOO-teh-rus) is the muscular, pear-shaped internal organ in a woman where a baby develops until birth.

* sperm are the tiny, tadpole-like cells males produce in their testicles. Sperm can unite with a female's egg to result eventually in conception.

* chromosome (KRO-mo-zom) is a unit or strand of DNA, the chemical substance that contains the genetic code to build and maintain a living being. Humans have 23 pairs of chromosomes, for a total of 46.

* embryo (EM-bree-o), in humans, is the developing organism from the end of the second week after fertilization to the end of the eighth week.

* cervix (SIR-viks) is the lower, narrow end of the uterus that opens into the vagina.

* menstrual cycle (MEN stroo-al SY-kul) culminates in menstruation (men-stroo-AY-shun), the discharging through the vagina of blood, secretions, and tissue debris from the uterus that recurs at approximately monthly intervals in females of reproductive age.

* fetus (FEE-tus) is the term for an unborn human after it is an embryo, from nine weeks after fertilization until childbirth.

* vagina (vah-JY-nah) is the canal, or passageway, in a woman that leads from the uterus to the outside of the body.

* cesarean section (si-ZAR-ee-an SEK-shun) is the surgical incision of the walls of the abdomen and uterus to deliver offspring in cases where the mother cannot deliver through the vagina.

* anemia (uh-NEE-me-uh) is a blood condition in which there is decreased hemoglobin in the blood and, usually, fewer than normal numbers of red blood cells.

* hemorrhoids (HEM-o-roidz) are a mass of dilated veins in swollen tissue at the margin of the anus or nearby within the rectum.

* stretch marks are stripes or lines on the skin (such as on the hips, abdomen, and breasts) from excessive stretching and rupture of elastic fibers, especially due to pregnancy or obesity.

* varicose veins (VAR-i-kose VAYNZ) are abnormally swollen or dilated veins.

* placenta (pluh-SEN-ta) is an organ that provides nutrients and oxygen to a developing baby; it is located within the womb during pregnancy.

* fallopian tubes (fa-LO-pee-an tubes) are the two slender tubes that connect the ovaries and the uterus in females. They carry the ova, or eggs, from the ovaries to the uterus.

* abdomen (AB-do-men), commonly called the belly, is the portion of the body between the thorax (THOR-aks) and the pelvis.

* ultrasound, also called a sonogram, is a diagnostic test in which sound waves passing through the body create images on a computer screen.

* convulsions (kon-VUL-shuns), also called seizures, are sudden bursts of disorganized electrical activity that interrupt the normal functioning of the brain, often leading to uncontrolled movements and sometimes a temporary change in consciousness.

* coma (KO-ma) is an unconscious state, like a very deep sleep. A person in a coma cannot be awakened, and cannot move, see, speak, or hear.

* amniotic sac (am-nee-AH-tik SAK) is the sac formed by the amnion, the thin but tough membrane that lines the outside of the embryo in the uterus and is filled with fluid to cushion and protect the embryo as it grows.

* prenatal (pre-NAY-tal) means existing or occurring before birth, with reference to the fetus.

* contraception (kon-tra-SEP-shun) is the deliberate prevention of conception or impregnation.

Disclaimer:   This information is not a tool for self-diagnosis or a substitute for professional care.

(MLA 8th Edition)