September 23, 2011

Pregnancy is a time of great anticipation and immense physiologic change for a woman. A woman’s body goes through significant anatomic and hormonal changes during pregnancy in order to support the development of her growing baby. The estimated date of delivery or “due date” is generally 40 weeks after the first day of the last menstrual period. Ultrasound can also be used to establish the dating of the pregnancy if the last menstrual period is uncertain. Pregnancy is divided into trimesters for the purposes of discussing common milestones and problems at each period of development. Maternity care providers often refer to the “gestational age” in weeks during ongoing care; gestational age includes the 2 weeks prior to conception.

The first trimester includes the weeks between the last menstrual period and 12 weeks of pregnancy. Conception occurs in the fallopian tube 1-5 days after an ovum (egg) is released from the ovary and is fertilized by a sperm. The first division of cells occurs 12 hr later and the fertilized egg continues to divide every 12-15 hr thereafter. When the fertilized egg drops out of the fallopian tube into the uterus it contains hundreds of cells. Implantation of the fertilized ovum (now called a blastocyst) occurs 4-8 days after ovulation. The corpus luteum, a structure on the ovary that is a remnant from the ovarian follicle that released the egg, secretes the female hormone, progesterone. This hormone prevents the endometrial (uterine) lining from shedding, thus allowing the blastocyst to remain implanted in the uterus. Occasionally, a fertilized ovum will not be able to exit the fallopian tube. This results in a condition called ectopic pregnancy. Ectopic pregnancies put the mother at significant risk for intra-abdominal bleeding as the fallopian tube is not designed to support a developing embryo.

Within 12 days after conception, the placenta, a collection of both embryonic and maternal cells, begins to form. This organ exchanges nutrients from the maternal blood supply for waste products from the fetus. The developing embryo attaches to the placenta via the umbilical cord. The placenta and ovaries all contribute to increased levels of progesterone, estrogen, and betahuman chorionic gonadotropin (beta-hCG, the “pregnancy hormone”) that are necessary to maintain a normal pregnancy.

At 5 weeks’ gestation (3 weeks after conception) the embryo is in. long and has developed three separate cell layers. One layer develops into the neural tube (brain, spinal cord, nerves, and backbone). A second layer becomes the heart and blood vessels, bones, muscles, and genitourinary system. The heart begins to beat 21-22 days after conception. The third (inner) layer develops into the urinary bladder and esophagus, stomach, and bowel.

Three weeks later the embryo is in. long and the arms and legs have begun to develop. Transvaginal ultrasound can easily detect an embryo with a heartbeat at this time.

At 8 weeks’ gestation (5 weeks after conception) the embryo is 1— in. long. The beginnings of all major organs are formed and the skeleton and ears are beginning to develop.

At the end of the first trimester, or 11-14 weeks’ gestation (9-12 weeks since conception), the embryo is 3 in. long and weighs 1oz. All organ systems are in place and the brain, nerves, and muscles start to function. The embryo has begun to make small movements although the woman is not able to feel them due to its small size. The genitalia now have recognizable male or female characteristics.

The first trimester is a period of critical organ development. Infections and environmental toxins (tobacco products, illicit drugs, alcohol, medications, and chemical exposures) can cause abnormalities in development, particularly 3-8 weeks after conception (5-10 weeks’ gestation). Examples of problems that can occur include abnormal development of the brain and spinal cord (neural tube defects) and miscarriage. The risk of neural tube defects can be decreased by ensuring maternal intake of at least 400 mg of folic acid daily prior to conception. Over 80% of miscarriages occur in the first 12 weeks of pregnancy. They are most commonly caused by abnormal division of the embryonic cells or abnormalities of the fetal chromosomes that lead to abnormal organ development. Common symptoms of miscarriage include cramping and bleeding.

Many women experience nausea and vomiting, increased fatigue, frequent urination, breast tenderness, and dizziness during the first trimester. Nausea and vomiting are due to the hormonal changes, primarily increased progesterone and beta-hCG, that occur with early pregnancy. Up to 70% of women experience these symptoms, which typically begin at 6-8 weeks’ gestation and diminish after 13-16 weeks. Fatigue is due to increased metabolic demands on the female body. A pregnant woman begins to produce more blood cells and her heart rate increases to accommodate the need to pump blood to the placenta and developing embryo. The increasing size of the uterus and increased efficiency of the maternal kidneys both contribute to urinary frequency, a condition which eases during the second trimester. Increased maternal estrogen and progesterone both cause breast tenderness, enlargement, and tingling. These hormones also promote further breast development throughout the pregnancy to support lactation. Dizziness due to hormonal changes as well as changes in heart rate and blood volume may also be worsened by fatigue and nausea. Concerning symptoms that should be reported to the maternity care provider during the first trimester of pregnancy include vaginal bleeding and cramping, abdominal pain, and painful urination.

The second trimester includes 13-27 weeks of gestation (11-25 weeks after conception). At 15-18 weeks’ gestation (13-16 weeks after conception) the fetus is 3in. long and weighs 7 oz. The eyebrows and scalp hair are beginning to develop. By 19-22 weeks’ gestation (17-20 weeks after conception) the fetus is 7in. long and weighs 1 lb. At this time, vernix, a white creamy substance, forms on the skin in order to protect it from the amniotic fluid. The fetal kidneys have begun to make urine and all internal organs are formed. The fetus is able to hear and react to sound. At the end of the second trimester (21-25 weeks after conception or 23-27 weeks’ gestation) the fetus has little body fat. It is 11-15 in. long and weighs 1-2-—2 lb. The lungs have begun to secrete surfactant, a chemical which will prepare the fetal lungs to breathe air after birth.

From a maternal standpoint, the second trimester is often referred to as the “golden period.” Fatigue and nausea have improved and the body has not yet changed enough to feel awkward. Most women begin to experience fetal movement, described as “fluttering” or “tapping” between 18 and 20 weeks’ gestation. The uterus is rapidly increasing in size and will be measured at each prenatal visit. It typically reaches the level of the maternal umbilicus by 20 weeks’ gestation and increases by 1 cm per week thereafter. Lower blood pressure and increased heart rate may predispose the mother to lightheadedness with rapid changes in position. There may be an increase in vaginal discharge due to increased hormonal levels. Common physical complaints at this time include leg cramps and low back pain. Slowing of bowel motility due to progesterone can cause increased heartburn and constipation. Anticipated maternal weight gain during this portion of pregnancy is 1 lb per week.

The third trimester encompasses 28-40 weeks’ gestation (26-38 weeks after conception). A fetus between 28 and 31 weeks’ pregnancy is able to open its eyes. The lungs are more developed but often require support for breathing if born prematurely. At this stage, the fetus weighs 2-3 lb and is 12-16 in. long.

Between 32 and 36 weeks’ gestation, the fetal lungs continue to mature and the sucking reflex, required for feeding, is improving. Premature infants at this stage of development vary in their need for support of temperature, feeding, and breathing. These fetuses range from 3 to 6 lb and are 16-19 in. long.

Typical maternal symptoms during the third trimester include shortness of breath due to increased pressure of the growing fetus and uterus on the diaphragm. Some pregnant women experience sciatic nerve pain (sharp pain or numbness in the buttock radiating into the leg) due to pressure of the uterus on this nerve. Sharp or stabbing pain in the vaginal area is not uncommon as the cervix begins to change in preparation for childbirth. This is generally not concerning unless accompanied by abdominal pain or bleeding. Increased urination and difficulty finding a comfortable sleeping position result in maternal sleep disturbances.

At 37-40 weeks’ gestation the fetus is considered full term. Less than 3% of women will deliver on their due date at 40 weeks’ gestation. Average weight at the time of delivery is 6-9 lb with a length of 18-21 in. Many pregnancies may continue for 1-2 weeks after the due date. Labor may commence at any time. The baby is fully developed, and if healthy, is ready for delivery and life outside the mother. A pregnancy is not considered abnormally prolonged until it has exceeded 42 weeks.

SEE ALSO: Labor and delivery, Miscarriage, Neural tube defects, Pregnancy testing, Prenatal care

Suggested Reading

  • Baxley, E. G. (2001). Physiologic changes and common discomforts of pregnancy. In S. D. Ratcliffe (Ed.), Family practice obstetrics (2nd ed.). Philadelphia: Hanley & Belfus.
  • Johnson, R. V. (Ed.). (1994). Mayo Clinic complete book of pregnancy and baby’s first year. New York: William Morrow.
  • Nilsson, L., & Hamberger, L. (1990). A child is born. New York: Bantam Doubleday Dell.


  • fetus development 26-38 weeks

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