Fetal development: The 1st trimester

You're pregnant. Congratulations! You'll undoubtedly spend the months ahead wondering how your baby is growing and developing. What does your baby look like? How big is he or she? When will you feel the first kick?

Fetal development typically follows a predictable course. Find out what happens during the first trimester by checking out this weekly calendar of events. Keep in mind that measurements are approximate.

Weeks 1 and 2: Getting ready

It might seem strange, but you're not actually pregnant the first week or two of the time allotted to your pregnancy. Yes, you read that correctly!

Conception typically occurs about two weeks after your last period begins. To calculate your estimated due date, your health care provider will count ahead 40 weeks from the start of your last period. This means your period is counted as part of your pregnancy — even though you weren't pregnant at the time.

Week 3: Fertilization

The sperm and egg unite in one of your 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, you might have multiple zygotes.

The zygote typically has 46 chromosomes — 23 from the biological mother and 23 from the biological father. These chromosomes help determine your baby's sex and physical traits.

Soon after fertilization, the zygote travels down the fallopian tube toward the uterus. At the same time, it will begin dividing to form a cluster of cells resembling a tiny raspberry — a morula.

Week 4: Implantation

The rapidly dividing ball of cells — now known as a blastocyst — has begun to burrow into the uterine lining (endometrium). 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, which will nourish your baby throughout the pregnancy.

Week 5: Hormone levels increase

The fifth week of pregnancy, or the third week after conception, the levels of HCG hormone produced by the blastocyst quickly increase. This signals your ovaries to stop releasing eggs and produce more estrogen and progesterone. Increased levels of these hormones stop your menstrual period, often the first sign of pregnancy, and fuel the growth of the placenta.

The embryo is now made of three layers. The top layer — the ectoderm — will give rise to your baby's outermost layer of skin, central and peripheral nervous systems, eyes, and inner ears.

Your baby's heart and a primitive circulatory system will form in the middle layer of cells — the mesoderm. This layer of cells will also serve as the foundation for your baby's bones, ligaments, kidneys and much of the reproductive system.

The inner layer of cells — the endoderm — is where your baby's lungs and intestines will develop.

Week 6: The neural tube closes

Growth is rapid this week. Just four weeks after conception, the neural tube along your baby's back is closing. The baby's brain and spinal cord will develop from the neural tube. The heart and other organs also are starting to form.

Structures necessary to the formation of the eyes and ears develop. Small buds appear that will soon become arms. Your baby's body begins to take on a C-shaped curvature.

Week 7: Baby's head develops

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

Lower limb buds that will become legs appear and the arm buds that sprouted last week now take on the shape of paddles.

Week 8: Baby's nose forms

Eight weeks into your pregnancy, or six weeks after conception, your baby's lower limb buds take on the shape of paddles. Fingers have begun to form. Small swellings outlining the future shell-shaped parts of your baby's ears develop and the eyes become obvious. 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 crown to rump — about half the diameter of a U.S. quarter.

Week 9: Baby's 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. Your baby's head is large but still has a poorly 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 — the diameter of a U.S. penny.

Week 10: Baby's elbows bend

By the 10th week of pregnancy, or eight weeks after conception, your baby's head has become more round.

Your baby can now bend his or her elbows. Toes and fingers lose their webbing and become longer. The eyelids and external ears continue to develop. The umbilical cord is clearly visible.

Week 11: Baby's genitals develop

At the beginning of the 11th week of pregnancy, or the ninth week after conception, your baby's head still makes up about half of its length. However, your baby's body is about to catch up.

Your baby is now officially described as a fetus. This week your baby's face is broad, the eyes widely separated, the eyelids fused and the ears low set. Buds for future teeth appear. Red blood cells are beginning to form in your baby's liver. By the end of this week, your baby's external genitalia will start developing into a penis or a clitoris and labia majora.

By now your baby might measure about 2 inches (50 millimeters) long from crown to rump — the length of the short side of a credit card — and weigh almost 1/3 ounce (8 grams).

Week 12: Baby's fingernails form

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

By now your baby might be about 2 1/2 inches (61 millimeters) long from crown to rump — the length of the short side of a U.S. bill — and weigh about 1/2 ounce (14 grams).