Fetal Development Timeline
It's confusing, but doctors typically measure the start of pregnancy as the first day of your last menstrual period. This is called the 'menstrual age' and is about two weeks ahead of when conception actually occurs, so throughout the timeline, fetal development lags 2 weeks behind your week of pregnancy.

4 Weeks
Cells are already specializing, with the outer layer of the fertilized egg becoming the nervous system, skin, and hair. The inner layer will be the breathing and digestive organs. The middle layer will be the skeleton, muscles, circulatory system, kidneys, and sex organs. Home pregnancy tests are often positive now.

6 Weeks
The embryo looks like a tadpole. It's the size of a BB pellet from crown to rump (crown-to-rump length is used because the baby's legs are usually not well developed yet and are bent and hard to measure). Ultrasound usually detects a heartbeat by now. Between 17 and 56 days the embryo is most susceptible to drugs, disease, and other factors that interfere with normal growth.

12 Weeks
The fetus measures about 6 to 7 cm (2.3 to 2.75 inches) from crown to rump. The fetus will start to make its own movements. Your chance of miscarriage drops considerably after this week.

20 Weeks
Your baby weighs about 5 ounces (300 grams). The nervous system is starting to function. The external genitalia can be used as a means for you to see if it's a boy or a girl. It 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."

24 Weeks
Your baby weighs about 1.4 pounds (about 630 grams). Baby responds to sounds by moving or increasing its pulse. You may notice jerking motions if baby hiccups.

28 Weeks
Your baby weighs about 2 pounds 6 ounces (about 1100 grams). There's a good chance the baby would survive if born prematurely now. Ask your doctor about preterm labor warning signs. Register for birthing classes. Baby changes position frequently at this point in pregnancy.

32 Weeks
Often on the move, your baby weighs almost 4 pounds (1.8 kilograms). A layer of fat is forming under the skin. Baby 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.

36 Weeks
Babies differ in size, depending on many factors (such as whether it is a boy or a girl, the number of babies you are carrying, the size of the parents, etc.), so your baby's overall rate of growth is as important as the actual size. On average, the baby is about 12.5 inches (32 centimeters) from crown to rump and weighs about 5.5 pounds (2.5 kilograms). The brain has been developing rapidly. Lungs are nearly fully developed. Baby moves less due to tight space. The head is usually positioned down into the pelvis by now. A term pregnancy begins at 37 weeks; baby is ready! Be sure you have baby gear for the first few weeks.

Medical Review: Reviewed by Louise Chang, MD, April 2007.

SOURCE DOCUMENTATION: Williams Obstetrics 21st edition.

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