The length of pregnancy, from fertilization of the ovum to birth, is about 38 weeks or 266 days. In practice, it is calculated as approximately 280 days or 40 weeks from the first day of the last menstrual period (LMP).
For study purposes, pregnancy is divided into 3-month periods (trimesters), during which defined changes can be observed in the fetus.
Childbirth or parturition occurs in three stages: (1) onset of regular uterine contractions and dilation of the cervix; (2) expulsion of the fetus; (3) delivery of the placenta and fetal membranes. The third stage is followed by contraction of the uterus and control of bleeding. The factors that start labor are not completely understood, but it is clear that the hormone oxytocin from the posterior pituitary gland and other hormones called prostaglandins are involved.
The term gravida refers to a pregnant woman. A prefix may be added to show the number of pregnancies, such as primigravida, meaning a woman pregnant for the first time, or a number may be used, such as gravida 1, gravida 2, and so forth. The term para refers to a woman who has given birth. This means the production of a viable infant (500 g or more or over 20 weeks' gestation) regardless of whether the infant is alive at birth or whether the birth is single or multiple. Again, prefixes or numerals are used to indicate the number of such pregnancies.
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