Pregnancy is measured in trimesters from the first day of your last menstrual period, totaling 40 weeks. The first trimester of pregnancy is week 1 through week 12, or about 3 months. The second trimester is week 13 to week 27. And the third trimester of pregnancy spans from week 28 to the birth.

Your baby will change from week to week. For more information about how your baby is changing each month and about what tests you might think about having, see the Interactive Tool: From Embryo to Baby in 9 Months .

First trimester

During the week after fertilization, the fertilized egg grows into a microscopic ball of cells (blastocyst), which implants on the wall of your uterus. This implantation triggers a series of hormonal and physical changes in your body.

The third through eighth weeks of growth are called the embryonic stage, during which the embryo develops most major body organs. During this process, the embryo is especially vulnerable to damaging substances, such as alcohol, radiation, and infectious diseases.

Having reached a little more than 1 in. (2.54 cm) in length by the ninth week of growth, the embryo is called a fetus. By now, the uterus has grown from about the size of a fist to about the size of a grapefruit.

The first trimester is a time of amazing development . The embryo starts out looking like a tiny seed, then a tadpole with a tail, and then more human.

Second trimester

If this is your first pregnancy, you'll begin to feel your fetus move at about 18 to 22 weeks after your last menstrual period. Although your fetus has been moving for several weeks, the movements have not been strong enough for you to notice until now. At first, fetal movements can be so gentle that you may not be sure what you are feeling.

If you've been pregnant before, you may notice movement earlier, sometime between weeks 16 and 18.

During this time, the fetus is still building up body fat and starting to put on a lot of weight. By the end of the second trimester , your fetus is about 10 in. (25.4 cm) long and weighs about 1.5 lb (680 g).

Third trimester

The third trimester of pregnancy spans from week 28 to the birth. Although your due date marks the end of your 40th week, a full-term pregnancy can deliver between week 37 and week 42. During this final trimester, your fetus grows larger and the body organs mature. The fetus moves frequently, especially between the 27th and 32nd weeks.

After week 32, a fetus becomes too big to move around easily inside the uterus and may seem to move less. At the end of the third trimester , a fetus usually settles into a head-down position in the uterus. You will likely feel some discomfort as you get close to delivery.