Fetal development: The first trimester

Fetal development starts soon after conception. Find out how your baby grows and develops during the first trimester.

You’re pregnant. Congratulations! As your pregnancy goes on, you may wonder how your baby is growing and developing. Use this information to follow along with what’s happening week by week during the first 12 weeks of pregnancy, called the first trimester.

Weeks 1 and 2: Getting ready

It might seem odd, but you’re actually not pregnant the first week or two of what’s counted as your pregnancy’s 40 weeks. Conception typically happens about two weeks after the last period begins. But to find your estimated due date, your healthcare professional counts ahead 40 weeks from the start of your last period. That means the period is counted as part of your pregnancy timeline even though you weren’t pregnant at the time.

Week 3: Fertilization

The sperm and egg unite in one of the fallopian tubes to form a one-celled entity called a zygote. If more than one egg is released and fertilized, or if the fertilized egg splits into two, the result may be multiple zygotes and a twin pregnancy.

The zygote typically has 46 chromosomes. That includes 23 from each biological parent. The chromosomes help determine the baby’s sex and physical traits.

Soon after fertilization, the zygote travels down the fallopian tube toward the uterus. At the same time, it begins dividing to form a cluster of cells called a morula. A morula looks like a tiny raspberry.

Week 4: Implantation

Once it reaches the uterus, the morula becomes what’s called a blastocyst. The rapidly dividing ball of cells begins to burrow into the uterine lining. This process is called implantation.

Within the blastocyst, the inner group of cells will become the embryo. The outer layer will give rise to part of the placenta. The placenta nourishes the unborn baby throughout pregnancy.

Week 5: Hormone levels rise

The fifth week of pregnancy, or the third week after conception, the level of the pregnancy hormone human chorionic gonadotropin, also called HCG, quickly rises. That tells the ovaries to stop releasing eggs and make more of the hormones estrogen and progesterone. Higher levels of estrogen and progesterone stop the menstrual period and fuel growth of the placenta.

The embryo is now made of three layers. The top layer, called the ectoderm, will become the outer layer of skin, central and peripheral nervous systems, eyes, and inner ears.

The heart and a primitive circulatory system will form in the middle layer of cells, called the mesoderm. This layer of cells also will serve as the foundation for the bones, ligaments, kidneys and much of the reproductive system.

The inner layer of cells, called the endoderm, is where the lungs and intestines will develop.

Week 6: The neural tube closes

The sixth week of pregnancy, or four weeks after conception, the neural tube along your baby’s back is closing. The brain and spinal cord develop from the neural tube. The heart and other organs also start to form.

Structures needed to form the eyes and ears develop. Small buds appear that will soon become arms. Your baby’s body begins to take on a C-shaped curve.

Week 7: The head develops

Seven weeks into pregnancy, or five weeks after conception, your baby’s brain and face are growing. Depressions that will give rise to nostrils become visible. The start of the eyes’ retinas form.

Lower limb buds appear that will become legs. The arm buds now take the shape of paddles.

Week 8: The nose forms

Eight weeks into your pregnancy, or six weeks after conception, your baby’s leg buds take the shape of paddles. Fingers have begun to form. Small swellings outlining the future shell-shaped parts of the ears develop, and the eyes become noticeable. The upper lip and nose have formed. The trunk and neck begin to straighten.

By the end of this week, your baby might be about 1/2 inch (11 to 14 millimeters) long from the top of the head to the bottom of the tailbone. This measurement is called crown to rump.

Week 9: Toes appear

In the ninth week of pregnancy, or seven weeks after conception, your baby’s arms grow, and elbows appear. Toes are visible and eyelids form. The head is large, and it doesn’t have a clearly formed chin.

By the end of this week, your baby might be a little less than 3/4 inch (16 to 18 millimeters) long from crown to rump.

Week 10: The elbows bend

By the 10th week of pregnancy, or eight weeks after conception, your baby’s head has become rounder. And the elbows can now bend. Toes and fingers lose their webbing and become longer. The eyelids and the outer parts of the ears continue to develop.

Week 11: Genitals develop

At the 11th week of pregnancy, or the ninth week after conception, your baby now is called a fetus. At this point in pregnancy, the face is broad, the eyes widely separated, the eyelids fused, and the ears are set low on the side of the head. Buds for future teeth appear. Red blood cells are beginning to form in the liver. By the end of this week, the outer genitals start developing.

By now, your baby might measure about 2 inches (50 millimeters) long from crown to rump. Your baby may weigh about 1/3 ounce (8 grams).

Week 12: Fingernails form

Twelve weeks into your pregnancy, or 10 weeks after conception, your baby is sprouting fingernails. Your baby’s face has taken on a more developed profile. The intestines are in the abdomen.

By now, your baby might be about 2 1/2 inches (61 millimeters) long from crown to rump and weigh about 1/2 ounce (14 grams).

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Fetal development: The first trimester