4th Week

The embryo, has multiplied to more than 250 cells by Day 6 of development. 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.

6th Week

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.

12th Week

The fetus measures about 1.5 to 3 inches from crown to rump and weighs approximately one and one-half ounces. Its organs are all present, and will continue to develop through the second and third trimesters. Since the most critical development has taken place, your chance of miscarriage drops considerably after this week.

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20th Week

Your baby is now big enough that you can see if it's a boy or a girl on an ultrasound. 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 a little over 1 pound (about half a kilogram). Baby responds to sounds by moving or increasing its pulse. You may notice jerking motions if baby hiccups. Some babies can survive if delivered after the 23rd week.

28 Weeks

Your baby weighs about 2 pounds 4 ounces (about 1 kilogram). Brain wave patterns appear like a full-term newborn. The branches of the lungs are developing, so 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 21 inches (53.3 centimeters) from head to toe and weighs almost 6.5 pounds (3 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.