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Although most people think of pregnancy as lasting nine months, a full term pregnancy is closer to ten months -- 38 to 40 weeks. The official start of pregnancy is the first day of a woman’s last menstrual period (LMP), which is usually three to four weeks before she is “officially” pregnant. Each of the three trimesters or stages of pregnancy is about 13 weeks long. Each stage consists of specific medical and physiological developments.

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Before Conception

Each month, the ovary releases a group of eggs, oocytes, for ovulation. These eggs develop within small fluid-filled sacs called follicles. In most cases, only one egg matures and becomes a dominant follicle. It suppresses the other follicles, which deteriorate and stop growing. Approximately two weeks before the woman’s menstrual cycle begins, the surviving follicle opens, releasing the egg from the ovary during the ovulation process. Once ovulation occurs, the ruptured follicle becomes the corpus luteum, a structure that secretes estrogen. It also secretes progesterone, a steroid that helps prepare the lining of the uterus for the implanting of the embryo.

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Fertilization

Conception occurs when sperm fertilizes the egg. This is the first event in the first trimester of pregnancy. Once fertilized, the egg begins its journey through the fallopian tube to the uterus. About a week later, the fertilized egg has become a group of around 100 cells called a blastocyst. It attaches itself to the uterine wall and begins to form the embryo and placenta. The released hormones signal to the uterus that there is a pregnancy. Instead of shedding its lining, the uterus keeps it, which prevents the monthly menstrual period. This is one of the first external signs of pregnancy.

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First Trimester: The First 13 Weeks

A woman’s body goes through many changes during the first 13 weeks of pregnancy. She releases hormones that affect every organ. Most women report a feeling of tiredness or fatigue, especially in the early weeks. Altered hormonal levels can cause mood swings, headaches, constipation, acne, morning sickness, insomnia, and even back pain. Tender, swollen breasts generally occur shortly after conception, but this symptom usually subsides within a few weeks. Increased urination, heartburn, and food aversions are also common symptoms of early pregnancy. The chance for miscarriage drops considerably after the first trimester.

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Fetal Development in the First Trimester

During the first trimester, the fetus develops rapidly. In the first month, an amniotic sac develops, along with the placenta. A face starts to develop, as well as a mouth, throat, and lower jaw. The brain, spinal cord, and internal organs start to form, and arms and legs begin to grow. In the third month, fingers and toes develop, along with the reproductive organs. By the end of the first trimester, the fetus is three or four inches long and weighs around one ounce. It is completely formed.

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Second Trimester: Weeks 13 Through 27

During the next 13 weeks, many of the symptoms the woman experienced in the first trimester subside. As the uterus expands to accommodate the growing fetus, the mother starts to “show.” Weight gain is common due to the baby’s growth and an increase in the mother’s blood and fluid volumes. She may experience body aches and some swelling of the feet, ankles, and hands. Stretch marks may appear on the stomach, buttocks, and breasts. Some women experience lightheadedness due to the growing uterus putting pressure on blood vessels and causing a drop in blood pressure. Braxton Hicks contractions, mild contractions that feel similar to a tightening in the abdomen, may begin in the second trimester.

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Fetal Development in the Second Trimester

In the fourth, fifth, and sixth months of pregnancy, the fetus has functioning organs, nerves, and muscles. The bones are hardening, but the skin is still transparent and thin. At about 14 weeks, doctors may be able to identify the sex of the baby. The spleen is now producing red blood cells. Toward the end of the second trimester, the baby begins to move around. Fingerprints and footprints form. By week 25, the fetus can respond to familiar voices. Lung development occurs towards the end of the second trimester. The fetus weighs around two pounds in week 27.

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Third Trimester: Weeks 28 Through 40

Swelling of the feet, ankles, hands, and even the face may occur. In this stage of pregnancy, the baby is putting more pressure on the mother’s organs. She may begin to feel shortness of breath or a more frequent urge to go to the bathroom. Leg cramps are common. Some women report sleep disturbances in the third trimester. The breasts may leak colostrum, the watery fluid that is the first stage of breast milk. The obstetrician will closely monitor the mother’s blood pressure, weight, and urine, and will likely increase the frequency of visits. Braxton-Hicks contractions may increase, though not all expectant mothers experience them.

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Fetal Development in the Third Trimester

By the third trimester, the fetus can not only see and hear but also has learned to suck its thumb and cry. A thin protective coating called vernix covers the entire body. The bones become sturdier, but the skull remains soft to make passage through the birth canal easier. By week 36, the infant’s head may drop into the pelvic area. This process is called lightening. At the end of the third trimester, the fetus usually weighs between six and nine pounds and is 19 to 21 inches long.

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The First Two Stages of Labor

The start of regular contractions signifies the beginning of labor. These contractions cause the cervix to open or dilate. It becomes softer and thins, a process called effacement. These conditions make it easier for the baby to move into the birth canal. The first stage of labor is the longest, consisting of two phases: early labor and active labor. During early labor, the contractions are mild and irregular. The body discharges the mucus plug covering the opening of the cervix during this phase. As active labor begins, the contractions become stronger and occur at more regular intervals, then start occurring closer together and increasing in intensity. The baby is born during the second stage of labor.

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The Third Stage of Labor

Once the baby arrives, the body delivers the placenta as well. Physicians will examine the placenta to ensure no fragments are left in the uterus, as these can cause bleeding and infection. Mild contractions will continue after the placenta delivery and can be painful. A member of the delivery team massages the uterus to reduce bleeding and cramping and to promote its contraction back to nonpregnant size. The physician will also stitch any vaginal tears, if needed.

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Disclaimer

This site offers information designed for educational purposes only. You should not rely on any information on this site as a substitute for professional medical advice, diagnosis, treatment, or as a substitute for, professional counseling care, advice, diagnosis, or treatment. If you have any concerns or questions about your health, you should always consult with a physician or other healthcare professional.