Gestation Period and Gestational Age

Encyclopedia of Childhood and Adolescence, Apr 06, 2001

The human gestation period--the period of time between conception and labor--is approximately forty weeks (280 days), measured from the first day of the mother's last menstrual period. A gestation period of thirty-eight weeks (266 days) is calculated for women who are pregnant by a procedure such as in vitro fertilization or artificial insemination that allows them to know their exact date of conception. The gestational period is divided into three major periods called trimesters. The first trimester last from weeks one through twelve, the second from weeks thirteen through twenty-seven, and the third from weeks twenty-eight to forty. The gestational age of a fetus or newborn infant corresponds to how far along it is in the gestation period, usually measured in weeks and days from the first day of the mother's last menstrual period.

The gestation period may also be calculated by an ultrasound examination, which is most reliable for this purpose between the eighth and eighteenth weeks of pregnancy. This is the method used by most doctors to determine a woman's due date and the gestational age of the fetus. Due dates for women who conceive naturally are only approximations and are precisely accurate for only one pregnancy out of twenty. Even estimations based on an ultrasound examination can be off by ten to fourteen days. Babies delivered anywhere from three weeks early to two weeks late are considered normal full-term babies. Knowledge of the gestational age is important in assessing whether or not a pregnancy is progressing normally and also in evaluating the health status of a newborn. Standard criteria for size, growth, and maturation are available for all infants, whether they are delivered prematurely, at term, or later than term, to determine whether their physical condition is appropriate for their gestational age (AGA). For a full-term infant, the length appropriate for gestational age is forty-eight to fifty-three centimeters and the weight is between 2700 and 4000 grams. Gestational age is also used as a baseline to identify babies who are at risk because of their small size--small for gestational age (SGA) or small for date (SFD). No matter how early or late they are delivered, infants are considered small for their gestational age (SGA) if their size and weight at birth are below the tenth percentile of the appropriate range. Such infants are at increased risk for numerous health problems, including short stature, certain infections, respiratory problems, and sudden infant death syndrome .

• Cherry, Sheldon. Understanding Pregnancy and Childbirth, 3rd ed. New York: Macmillan, 1992.
• Grunfeld, Nina. Pregnancy Week by Week. New York: Smithmark, 1995.