Pregnancy Trimester Overview
Pregnancy is divided into three phases, called trimesters. Each trimester has its own significant developmental milestones.
The first pregnancy trimester is the most fragile period, during which all major organs and systems in your baby's body are formed. Most birth defects and miscarriages occur during the first trimester.
During the second and third trimesters, the fetus is fully formed and grows and matures rapidly. The trimesters are divided as follows:
However, some doctors use the 42-weeks method, which is calculated as follows:
Talk to your doctor about which method he/she will use to discuss your pregnancy and prenatal care with you.
Amniotic sac - a thin-walled sac that surrounds the fetus during pregnancy. The sac is filled with amniotic fluid (liquid made by the fetus) and the amnion (the membrane that covers the fetal side of the placenta), which protects the fetus from injury and helps to regulate the temperature of the fetus.
Anus - the opening at the end of the anal canal.
Cervix - the lower part of the uterus that projects into the vagina. Made up of mostly fibrous tissue and muscle, the cervix is circular in shape.
Fetus - an unborn baby from the eighth week after fertilization until birth.
Placenta - an organ, shaped like a flat cake, that only grows during pregnancy and provides a metabolic interchange between the fetus and mother. (The fetus takes in oxygen, food, and other substances and eliminates carbon dioxide and other wastes.)
Umbilical cord - a rope-like cord connecting the fetus to the placenta. The umbilical cord contains two arteries and a vein, which carry oxygen and nutrients to the fetus and waste products away from the fetus.
Uterine wall - the wall of the uterus.
Uterus (also called the womb) - the uterus is a hollow, pear-shaped organ located in a woman's lower abdomen, between the bladder and the rectum, that sheds its lining each month during menstruation and in which a fertilized egg (ovum) becomes implanted and the fetus develops.
Vagina - the part of the female genitals, behind the bladder and in front of the rectum, that forms a canal extending from the uterus to the vulva.