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Pregnancy Week by Week

Stages of Pregnancy at a Glance

  • Pregnancy begins when a fertilized egg implants in the uterus.
  • Women may experience a wide range of common symptoms during pregnancy.
  • Fetuses grow and change dramatically throughout a pregnancy.

Women go through many changes during each stage of pregnancy. Many of the pregnancy symptoms they have and the changes they deal with are common to all healthy pregnancies. The stages of embryonic and fetal development also follow a common pattern.

Still, pregnancy can be confusing and sometimes mysterious. It’s normal to wonder what happens during each of the stages of pregnancy.

Whether you are pregnant, are a concerned partner or friend, or are just curious, you may have many questions. Here is a description of the stages of pregnancy and the changes that women go through during a healthy pregnancy.

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What Happens in the First Month of Pregnancy?

Pregnancy is divided into 3 trimesters. Each trimester is a little longer than 13 weeks. The first month marks the beginning of the first trimester.

 

Gestational Age
Pregnancy is measured using “gestational age.” Gestational age starts on the first day of a woman’s last menstrual period (LMP).


Gestational age can be confusing. Most people think of pregnancy as lasting nine months. And it’s true that a woman is pregnant for about nine months. But because pregnancy is measured from a woman’s last menstrual period — about 3-4 weeks before she is actually pregnant — a full-term pregnancy usually totals about 40 weeks from LMP — roughly 10 months.


Many women do not remember the exact date of their last menstrual period — that’s OK. The surest way to tell gestational age early in pregnancy is with ultrasound.

Weeks 1–2

These are the first two weeks of a woman’s menstrual cycle. She has her period.  About 2 weeks later, the egg that is most mature is released from the ovary — ovulation. Ovulation may happen earlier or later, depending on the length of a woman’s menstrual cycle. The average menstrual cycle is 28 days.

After it is released, the egg travels down a fallopian tube toward the uterus. If the egg meets a sperm, they combine to form one cell. This is called fertilization. Fertilization is most likely to occur when a woman has unprotected vaginal intercourse during the 6 days that lead into ovulation.

Weeks 3–4

The fertilized egg moves down the fallopian tube and divides into more and more cells. It reaches the uterus about 3–4 days after fertilization. The dividing cells then form a ball that floats free in the uterus for about 2–3 days.

Pregnancy begins when the ball of cells attaches to the lining of the uterus. This is called implantation. It usually starts about 6 days after fertilization and takes about 3–4 days to be complete.

Pregnancy does not always occur. Up to half of all fertilized eggs pass out of women’s bodies during regular menstruation before implantation is complete.

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Learn more about how pregnancy happens.

 

A Woman’s First Signs of Pregnancy
For many women, the first sign of pregnancy is a missed period. Most pregnancy tests will be positive by the time a woman has missed her period. Other early signs of pregnancy include fatigue, feeling bloated, frequent urination, mood swings, nausea, and tender or swollen breasts. Not all women have all of these symptoms, but it is common to have at least one of them. 



What Happens in the Second Month of Pregnancy?

EMBRYONIC DEVELOPMENT
The ball of cells develops into an embryo at the start of the sixth week. The embryonic stage of pregnancy will last about 5 weeks. During this time all major internal organs begin developing.

Weeks 5–6

  • The embryo is less than 1/5 inch (4–5 mm) long.
  • A very basic beating heart and circulatory system develop.
  • Buds for arms and legs develop.
  • The neural tube begins forming. The neural tube will later form the brain, spinal cord, and major nerves.
  • The bud of a tail develops.
  • The umbilical cord begins developing.

Pregnancy Week 6

Weeks 7–8

  • The embryo is 1/4 to 1/2 inch (7–14 mm) long.
  • The heart has formed.
  • Webbed fingers and toes develop.
  • The arms bend at elbows.
  • External ears, eyes, eyelids, liver, and upper lip have begun forming.
  • The sex organs are the same — neither female nor male — in all embryos until the seventh or eighth week. If a gene triggers the development of testes, the embryo develops as a male. If there is no trigger, the embryo develops ovaries and becomes female.

Pregnancy Week 8

PREGNANCY SYMPTOMS

The second month is often when pregnancy symptoms become very noticeable.  Common discomforts like breast tenderness, fatigue, frequent urination, heartburn, nausea, and vomiting usually get worse. A woman’s body produces extra blood during pregnancy, and her heart beats faster and harder than usual to carry the extra blood.

What Happens in the Third Month of Pregnancy?

FETAL DEVELOPMENT

Weeks 9–10

  • The embryo develops into a fetus after 10 weeks. It is 1–1.5 inches (21–40 mm) long.
  • The tail disappears.
  • Fingers and toes are longer.
  • The umbilical cord connects the abdomen of the fetus to the placenta.  The placenta is attached to the wall of the uterus. It absorbs nutrients from the woman’s bloodstream. The cord carries nutrients and oxygen to the fetus and takes wastes away from the fetus.

Pregnancy Week 10

Weeks 11–12

The fetus is now measured from the top of its head to its buttocks. This is called crown-rump length (CRL).

  • The fetus has a CRL of 2–3 inches (6–7.5 cm).
  • Fingers and toes are no longer webbed.
  • Bones begin hardening.
  • Skin and fingernails begin to grow.
  • Changes triggered by hormones begin to make external sex organs appear — female or male.
  • The fetus begins making spontaneous movements.
  • Kidneys start making urine.
  • Early sweat glands appear.
  • Eyelids are fused together.

Pregnancy Week 12

PREGNANCY SYMPTOMS

Many of the pregnancy symptoms from the first 2 months continue — and sometimes worsen — during the third month. This is especially true of nausea. A woman’s breasts continue growing and changing. The area around the nipple — the areola — may grow larger and darker. Women who are prone to acne may experience outbreaks.

Women do not usually gain much weight during the first 3 months of pregnancy — usually about 2 pounds. Women who are overweight or underweight may experience a different rate of weight gain. Talk with your health care provider about maintaining a healthy weight throughout pregnancy.

 

Miscarriage
Most early pregnancy loss — miscarriage — happens in the first trimester. About 15 percent of pregnancies result in early pregnancy loss during the first trimester.


Learn more about miscarriage.

What Happens in the Fourth Month of Pregnancy?

The fourth month marks the beginning of the second trimester.

FETAL DEVELOPMENT

Weeks 13–14

  • The fetus has a CRL of about 3 inches (8 cm).
  • The sex of the fetus can sometimes be seen by looking at external sex organs on an ultrasound.
  • Hair begins to grow.
  • The prostate gland begins developing in male fetuses.
  • Ovaries move down from the abdomen to the pelvic area in female fetuses.
  • The roof of the mouth is formed.

Pregnancy Week 14

Weeks 15–16

  • The fetus has a CRL of about 4.5 inches (12 cm).
  • Hundreds of thousands of eggs are forming in the ovaries in female fetuses.

PREGNANCY SYMPTOMS

Some of the early signs and symptoms of pregnancy begin to be relieved during the fourth month. Nausea is usually reduced. But other digestive problems — heartburn and constipation — may be troublesome. Breast changes — growth, soreness, and darkening of the areola — usually continue. It’s common for women to have shortness of breath or to breathe faster. Increased blood flow may lead to unpleasant pregnancy symptoms, such as bleeding gums, nosebleeds, or nasal stuffiness. Pregnant women also may feel dizzy or faint because of the changes to their blood and blood vessels.

What Happens in the Fifth Month of Pregnancy?

Weeks 17–18

  • The fetus has a CRL of 5.5–6 inches (14–15 cm).

Pregnancy Week 18

Weeks 19–20

  • The fetus has a CRL of about 6.5 inches (16 cm).  
  • Lanugo  — a fine downy hair — covers the body. 
  • The skin is also covered with vernix caseosa, a greasy material that protects the skin.
  • A uterus has formed in a female fetus.

PREGNANCY SYMPTOMS

Women usually feel fetal movements for the first time during the fifth month. It may feel like flutters or butterflies in the stomach. This is called quickening.

The pregnancy symptoms of the fourth month continue this month. Heartburn, constipation, breast changes, dizziness, shortness of breath, nosebleeds, and gum bleeding are common. Breasts may be as much as 2 cup sizes bigger by this time.

What Happens in the Sixth Month of Pregnancy?

FETAL DEVELOPMENT

Weeks 21–22

  • The fetus has a CRL of about 7 inches (18–19 cm).
  • Bone marrow starts making blood cells.
  • Taste buds begin to form.

Pregnancy Week 22

Weeks 23–24

  • The fetus has a CRL of about 8 inches (20 cm).
  • Eyebrows and eyelashes usually develop between weeks 23 and 26.

PREGNANCY SYMPTOMS

Pregnancy symptoms from the fourth and fifth month usually continue. Shortness of breath may improve. Breasts may start producing colostrum — tiny drops of early milk. This may continue throughout pregnancy.

Some women have Braxton-Hicks contractions. They feel like a painless squeezing of the uterus or abdomen. This is the uterus’s way of practicing for labor and delivery. Braxton-Hicks contractions are normal and not a sign of preterm labor. But women should check with their health care providers if they have painful or frequent contractions or if they have any concerns.

What Happens in the Seventh Month of Pregnancy?

FETAL DEVELOPMENT

Weeks 25–26

  • The fetus has a CRL of about 9 inches (23 cm).
  • The fetus develops more and more fat from now until the end of pregnancy.

Week 27–28

  • The fetus has a CRL of about 10 inches (25 cm).
  • Eyelids are usually fused together until about 28 weeks.

Pregnancy Week 28

PREGNANCY SYMPTOMS
A woman’s uterus continues expanding. Back pain is common. Pregnancy symptoms from earlier months continue. Dizziness may lessen.

What Happens in the Eighth Month of Pregnancy?

The eighth month marks the beginning of the third trimester.

FETAL DEVELOPMENT

Week 29–30

  • The fetus has a CRL of about 10.5 inches (27 cm).
  • Testes usually begin descending into the scrotum from the abdomen between weeks 30 and 34 in a male fetus. This is usually complete by 40 weeks.

Week 31–32

  • The fetus has a CRL of about 11 inches (28 cm).
  • Lanugo starts falling off.

PREGNANCY SYMPTOMS
Women often start feeling tired and have a more difficult time breathing as the uterus expands up. They may get varicose veins — blue or red swollen veins most often in the legs — or hemorrhoids — varicose veins of the rectum. Hemorrhoids can be painful and itchy and cause bleeding. Women may also get stretch marks where skin has been expanded. Braxton-Hicks contractions, heartburn, and constipation may continue. Women may urinate a bit when sneezing or laughing because of pressure from the uterus on the bladder. Hormones may make hair appear fuller and healthier.

What Happens in the Ninth Month?

FETAL DEVELOPMENT

Week 33–34

  • The fetus has a CRL of about 12 inches (30 cm).
  • The eyes have developed enough for pupils to constrict and dilate when exposed to light.
  • Lanugo is nearly all gone.

Week 35–36

  • The fetus has a CRL of about 12.5 inches (32 cm).
  • The fetus is considerably fatter, and the skin is no longer wrinkled.

PREGNANCY SYMPTOMS
The growing fetus places more and more strain on a pregnant woman’s body. Common pregnancy symptoms continue through the end of pregnancy, including fatigue, trouble sleeping, trouble holding urine, shortness of breath, varicose veins, and stretch marks. Some fetuses drop down into the lower part of the uterus during this month. This may relieve the woman’s constipation and heartburn that are common earlier in pregnancy. But some fetuses do not drop down until the very end of pregnancy.

What Happens in the Tenth Month?

FETAL DEVELOPMENT

Week 37–38

  • The fetus has a CRL of about 13–14 inches (34–36 cm).
  • The fetus has a firm grasp.

Week 39–40

Many women give birth around this time.

  • The average newborn weighs 7–8 lbs. and is between 18–22 inches (46–56 cm) long with legs extended.
  • Almost all of the vernix and lanugo are gone. It is common for newborns to have some lanugo that disappears over the first few months of life.

Pregnancy Week 40

PREGNANCY SYMPTOMS
By the end of pregnancy, the uterus has expanded from a woman’s pelvis to the bottom of her rib cage. Pregnancy symptoms in the tenth month largely depend on when the fetus drops down into lower part of the uterus in the pelvis.

Shortness of breath, heartburn, and constipation usually improve when the fetus drops. But the position of the fetus lower in the pelvis causes frequent urination and trouble holding urine.

The cervix will begin to open — dilate — to prepare for delivery. This may happen a few weeks before delivery, or it might start when a woman goes into labor. A woman may feel sharp pains in her vagina as the cervix dilates.

After the newborn is delivered, the placenta and other tissues also come out of the woman’s body. This is called the afterbirth.

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Pregnancy Week by Week