Week-by-Week Pregnancy Guide
Knowing what to expect during each stage of pregnancy can help reduce the stress and anxiety associated with having a baby. Read our week-by-week pregnancy guide to learn more about the symptoms moms-to-be can expect to experience during each trimester, as well as the weekly stages of normal fetal growth and development.
Trimester One - Weeks One through 12
The first trimester of pregnancy is an exciting time full of changes and adjustments. During the first several weeks of pregnancy, the fetus forms its most basic organ systems, and a woman's body undergoes complex changes that can cause an array of pregnancy symptoms. Understanding fetal development and first-trimester symptoms will help women get through the initial weeks of pregnancy.
Once the embryo is implanted on the uterine wall, the body begins producing hormones that can trigger symptoms including:
- Tender, swollen breasts
- Slight cramping or spotting
- Nausea and/or vomiting
- Food cravings or aversions
- Mood swings
- Increased body temperature
The second half of the first trimester represents a crucial time in a fetus's development, and a light at the end of the tunnel for women experiencing difficult first-trimester pregnancy symptoms. During this period, the fetus's limbs, organ systems, and sex organs are refined, and parents can look forward to finding out the fetus' sex.
At the beginning of the second half of the first trimester, the uterus continues to grow, causing the woman's waistline to thicken a bit. Weight gain is typically minimal at this point, since pregnancy hormones continue to cause nausea, constipation, food aversions and cravings, and other symptoms that tend to curb the appetite. Mood swings, which are caused by hormonal fluctuations, are also common and normal. As the placenta matures, it takes over production of some of these hormones, and symptoms diminish toward the end of the first trimester.
First Trimester Fetal Development
- Week One: When the sperm penetrates the egg, it forms a cell that divides and travels toward the uterus. The cell group implants onto the uterine wall around day six, at which point it is referred to as an embryo. Implantation signals the beginning of pregnancy.
- Week Two: During week two, the cells of the implanted embryo multiply and differentiate.
- Week Three: The fetus's brain, spinal cord, heart, and digestive tract begin to form.
- Week Four: Rudimentary blood vessels begin to form, and the heart begins to beat.
- Week Five: The circulatory system further develops and becomes functional. The digestive and respiratory systems, arm and leg buds, and eye and ear structures begin to form.
- Week Six: The fetus is about an eighth-of-an-inch long. Organs and facial features become more well defined. The neural tube closes, which will later differentiate the brain, spinal cord, and nerves.
- Week Seven: The fetus's limbs and eyes and ear buds, and the umbilical cord linking the fetus to the placenta, are visible.
- Week Eight: The fetus forms fingers and toes, and the facial features become more distinct.
- Week Nine: During the ninth week, the fetus may begin moving. Hair follicles and reproductive organs begin to form.
- Week Ten: All the vital organs, fingers, and toes are in place. The skeleton begins to form, and male fetuses begin producing testosterone.
- Week Eleven: The fetus begins to grow more rapidly, nourished by the growing placenta. Genitalia become discernable by the end of this week.
- Week Twelve: Week twelve marks the end of the first trimester. By this week, the fetus is nearly three inches long, and has developed finger and toenails.
Trimester Two - Weeks 13 through 27
Many women look forward to the second trimester. The unpleasant first trimester symptoms subside, and women can resume most of their normal activities. The fetus's rapid development and the growing bump may make the pregnancy seem more real, and risk of miscarriage is greatly reduced.
As symptoms of early pregnancy subside in the beginning of the second trimester, many women experience an increase in their energy level and appetite. Women also begin to "look pregnant" at this point. Many women enjoy a rosy glow, since their circulatory systems expand to increase blood flow to the fetus. Other skin changes, including varicose veins, stretch marks, and brownish marks on the face or abdomen, will generally fade in the months after giving birth. Women should be sure to get regular, moderate exercise to improve circulation and prevent swelling and unnecessary weight gain.
As the second trimester progresses, a woman's breasts enlarge, and she can expect to gain about four pounds per month for the rest of the pregnancy. Changes in circulation can cause breathing and gum problems, dizziness, and cramps. The growing uterus cramps the abdominal area, causing indigestion, heartburn, shortness of breath, and sleeping problems. Hormonal fluctuations can cause constipation and bladder and kidney infections, although mood swings and lethargy decrease.
In the second half of the second trimester, the pregnancy is in "full swing"; the "bump" is fully visible, and a fetus born at the end of this trimester has strong chance of survival. Changes to a woman's body, and development of the fetus, are considerable during this time.
Second Trimester Fetal Development
- Weeks 13: The fetus has fully functioning nerves, organs, and muscles, and it flexes and kicks considerably.
- Week 14: The fetus's hormone production increases and sex organs develop.
- Week 15: Skin begins to form. Bone and marrow that will comprise the skeleton continue to form.
- Week 16: Fetuses are able to make facial expressions, hiccup, and make a fist. Female fetuses are forming millions of eggs in the ovaries.
- Week 17: Fat stores that will protect the organs after birth begin to develop under the skin.
- Week 18: The fetus is able to hear and swallow.
- Week 19: The fetus can now make reflexive muscle movements. A fine layer of hair called lanugo, and a pasty white coating called vernix, cover the skin.
- Week 20: The skin thickens, and the limbs are well-developed. The fetus weighs a little over half a pound and is about six inches long.
- Week 21: During this week, the bone marrow takes over red blood cell production from the liver and spleen. The fetus begins swallowing small amounts of sugar in the amniotic fluid.
- Week 22: The fetus is now able to sense taste and touch. For male fetuses, the testes descend into the abdomen.
- Week 23: The fetus begins to practice breathing by drawing amniotic fluid into and out of the lungs. The skin continues to mature and fat deposits increase.
- Week 24: The fetus's fully developed inner ear gives the fetus a sense of balance, and it may adopt a regular sleeping position.
- Week 25: The fetus's sensory system becomes more refined.
- Week 26: The eyes are fully developed, although the lids remain closed.
- Week 27: The lungs, liver, and immune system continue to develop, and the fetus weighs about two pounds. With intensive medical attention, a fetus born at 27 weeks has about an 85 percent chance of survival.
Trimester Three - Weeks 28 through 40
At the beginning of the third trimester, pregnancy becomes a bit more uncomfortable for a woman, while anticipation of the birth increases. The fetus matures rapidly, fine-tuning its neurological and sensory systems in preparation for birth. In the third trimester, the fetus is able to survive outside the womb with medical support.
During the last part of the third trimester women tend to be uncomfortable because of the increase in hormone levels and weight gain at this stage in pregnancy. The abdominal cavity is quite cramped, and indigestion, heartburn, problems sleeping, frequent urination, and constipation increase. Women may experience back and hip pain because of the weight of the fetus. Others experience leg cramps, vaginal pain, and swelling and fluid retention. Some women experience Braxton Hicks contractions, pre-term contractions of the uterus. If symptoms or contractions become intense, contact your healthcare provider.
In the last half of the third trimester, the end is in sight. A pregnant woman prepares for birth and often looks forward to having her body back. As the due date approaches, fetal development involves finishing touches that allow the fetus to survive outside the womb.
Third Trimester Fetal Development
- Week 28: The fetus's eyes open, and the eye color is established. The fetus may sleep for about 30 minutes at a time.
- Week 29: As the fetus begins to store iron, calcium, and phosphorus, its bones grow stronger and its movements become more vigorous.
- Week 30: From weeks 30 to 37, the fetus gains about a half a pound each week. The fetus practices breathing by moving the diaphragm.
- Week 31: Male fetuses' testicles drop into the scrotum (female fetuses' clitoris is relatively prominent).
- Week 32: Lanugo begins to fall off the skin. Due to the cramped quarters in the uterus, fetal movements may become less forceful. Contact your healthcare provider if movements decrease.
- Week 33: The fetus can detect light, and its lungs are almost fully developed.
- Week 34: The vernix covering the skin becomes thicker to protect the fetus's skin during birth. The fetus weighs about four or five pounds.
- Weeks 35: The fetus continues to gain weight and to store fat. Its movements may be restricted to stretches, rolls, and wiggles.
- Week 36: The fetus has its first bowel movement.
- Week 37: As fat accumulates, the fetus's body becomes rounder. The fetus is considered full-term at the end of this week.
- Week 38: The fetus's brain and nervous system are working more and more precisely, and will continue to improve after birth.
- Week 39: In preparation for life outside the womb, the placenta provides the fetus with infection-preventing antibodies. The fetus has enough fat to maintain a constant body temperature.
- Week 40: This is the week of the due date. At birth, the average fetus weighs seven to eight pounds and is 19 to 21 inches long.
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