Pregnancy week by week - First Trimester

After fertilization and implantation, a baby is at first just an embryo: two layers of cells from which all the organs and body parts will develop. Growing quickly, your baby is soon about the size of a kidney bean and constantly moving. The heart is beating quickly and the intestines are forming. Your budding son or daughter's earlobes, eyelids, mouth, and nose are also taking shape.

In early pregnancy your baby grows dramatically, from a tiny dot to the size of a grape.

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.

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.

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 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.

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.

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 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.