Slideshow: Fetal Development Month by Month

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Fetal Growth: Conception to Birth

You're pregnant. Congratulations! Are you curious how big your developing baby is, what your baby looks like as it grows inside you, and when you'll feel it move? Take a peek inside the womb to see how a fetus develops from month to month.

Conception

If a sperm cell meets and penetrates an egg, it will fertilize it. This is known as conception. At this moment, the genetic makeup is complete, including the sex of the infant. Within about three days after conception, the fertilized egg, which is dividing rapidly into many cells, passes through the fallopian tube into the uterus, where it attaches to the uterine wall. The placenta, which will nourish the baby, also begins to form.

Fetal Development at 4 Weeks

At this point of development the structures that eventually form the face and neck are becoming evident. The heart and blood vessels continue to develop. And the lungs, stomach, and liver start to develop. A home pregnancy test is now positive.

Fetal Development at 8 Weeks

The baby is now a little over half an inch in size. Eyelids and ears are forming, and the tip of the nose is visible. The arms and legs are well formed. The fingers and toes grow longer and more distinct.

Fetal Development at 12 Weeks

The fetus measures about 2 inches and starts to make its own movements. You may start to feel the top of your uterus above the pubic bone. Your doctor may hear the baby's heartbeat with special instruments. The sex organs of the baby should start to become clear.

Fetal Development at 16 Weeks

The fetus now measures about 4.3 to 4.6 inches and weighs about 3.5 ounces. The top of your uterus should be felt about 3 inches below your belly button. The baby's eyes can blink and the heart and blood vessels are fully formed. The baby's fingers and toes should have fingerprints.

Fetal Development at 20 Weeks

The baby weighs about 10 ounces and is a little over 6 inches long. Your uterus should be at the level of your belly button. The baby can suck a thumb, yawn, stretch, and make faces. Soon -- if you haven't already -- you'll feel your baby move, which is called "quickening."

Time for an Ultrasound

An ultrasound is generally performed for all pregnant women at 20 weeks of gestation. During this ultrasound, the doctor will confirm that the placenta is healthy and attached normally and that your baby is growing properly in the uterus. The baby's heartbeat and movement of its body, arms, and legs can also be seen on the ultrasound. The gender of the baby can usually be determined at 20 weeks.

Shown here is a 2D ultrasound (inset) contrasted with a 4D ultrasound, both at 20 weeks.

Fetal Development at 24 Weeks

The fetus weighs about 1.4 pounds now. It responds to sounds by moving or increasing its pulse. You may notice jerking motions if it hiccups. With the inner ear fully developed, it may be able to sense being upside down in the womb.

Fetal Development at 28 Weeks

The fetus weighs about 2 pounds 6 ounces. It changes position frequently at this point in pregnancy. There's a good chance of survival if your baby is born prematurely now. Ask your doctor about preterm labor warning signs. Register for birthing classes. Birthing classes prepare you for many aspects of childbirth, including labor and delivery and parenting the newborn.

Fetal Development at 32 Weeks

Often moving, the fetus weighs almost 4 pounds. The baby's skin has fewer wrinkles as a layer of fat starts to form under the skin. It will gain up to half its birth weight between now and delivery. Ask your doctor how to do a fetal movement chart. Think about breastfeeding. Soon you may start leaking colostrum from your breasts, a yellowish fluid that precedes milk production. Visits to the doctor generally occur every two weeks at this stage of pregnancy.

Fetal Development at 36 Weeks

Babies differ in size, depending on many factors (such as gender, the number of babies being carried, and size of the parents), so your baby's overall rate of growth is as important as the actual size. On average, it's about 18.5 inches and weighs close to 6 pounds. The brain has been developing rapidly. Lungs are nearly fully developed. The head is usually positioned down into the pelvis by now. A pregnancy is considered "at term" once 37 weeks has been completed!

Birth!

A mother's due date marks the end of her 40th week. A pregnancy begins with implantation and the delivery date can be calculated using the first day of the last period. Based on this, pregnancy can last between 38 and 42 weeks with a full-term delivery occurring around 40 weeks. Some post-term pregnancies -- those lasting more than 42 weeks -- are not truly post-term. A common "cause" is an incorrect due date. For safety reasons, most babies are delivered by 42 weeks, inducing labor if necessary.