Pregnancy is measured in trimesters from the first day of your last menstrual period (LMP), totaling 40 weeks. The first trimester of pregnancy is week 1 through week 12, or about 3 months.

Early development

Your actual pregnancy begins at the time the egg is fertilized by a sperm cell. During the following week, 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 1in. 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.

See illustrations of the fetus at 9 and 12 weeks of development (11th and 14th weeks after last menstrual period).

Early signs of pregnancy

Your first sign of pregnancy may be a missed menstrual period. Other early signs of pregnancy, caused by hormonal changes, include:

• Fatigue.
• Breast tenderness.
• Increased urination.
• Fullness or mild aching in your lower abdomen.
• Nausea with or without vomiting, also known as morning sickness. For more information, see:

  Pregnancy: Dealing with morning sickness.

Additional changes related to pregnancy

Throughout your pregnancy, you may notice a number of mild to severe effects, including:

• Constipation, due to hormonal changes that slow down the normal function of your bowels. Iron in your prenatal vitamin can also cause constipation.
• Mood swings, which can be caused by hormonal changes, extreme fatigue, or the stress of expecting a new baby.
• Vaginal discharge changes. A thin, milky-white discharge (leukorrhea) is normal throughout pregnancy. Also, the tissues lining the vagina become thicker and less sensitive during pregnancy.
• Vaginal yeast infections, which are more common in pregnancy because of the increased levels of hormones. Call your health professional if you have symptoms of a vaginal yeast infection or bacterial vaginal infection (bacterial vaginosis).
• Vaginal bleeding. Spotting in early pregnancy may go away on its own, but it can be the start of a miscarriage. If you experience any vaginal bleeding during pregnancy, contact your health professional. For more information, see the topic Miscarriage.