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 1 in. (2.54 cm) in length by the ninth week of growth, the embryo is called a foetus.

By now, the uterus has grown from about the size of a fist to about the size of a grapefruit.

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 (morning sickness) with or without vomiting.