Gestational Age

gestational age, length of time that a fetus grows inside the mother’s uterus. Gestational age is related to the fetus’s stage of growth as well as its cognitive and physical development. The gestational age of a fetus is particularly important when determining the potential negative effects of a fetal exposure to toxins or infection and has a direct impact when planning appropriate medical treatment for such situations.

Gestational age is divided into two periods: embryonic and fetal. Preceded by the embryonic period, the fetal period begins at the gestational age of week 10 and continues until birth. Prenatal development benchmarks are linked to gestational ages. For example, at the gestational age of 7 to 8 weeks, all the vital organs have begun to form, along with the bones and cartilage. By the gestational age of weeks 9 to 13, the genitalia have formed, and the entire fetus weighs about 1 oz (0.06 pounds). By weeks 21 to 23, the fetus’s eyes have developed, and the fetal heartbeat can be heard by stethoscope. By 40 weeks, fetal development is complete. Hence, a normal pregnancy has a gestational range of 38 to 42 weeks, with 40 weeks generally considered full-term pregnancy. Infants born at a gestational age of less than 38 weeks are considered premature and are susceptible to increased risks of illness and death.

In general, gestational age is measured by the number of weeks that have passed since the first day of the mother’s last menstrual period, though there is a potential for inaccuracies using this method, due to variations in ovulation dates. Other methods employed to accurately determine gestational age include ultrasound, whereby growth can be determined through measurements of the head and abdomen. After birth, a newborn’s gestational age can be measured using the Ballard Scale (or New Ballard Score), a modified version of the Dubowitz Scale. The Ballard Scale involves an examination of the neuromuscular and physical maturity of the newborn, with assessment of six criteria in each area. The Dubowitz Exam also focuses on neurological and physical maturity of the newborn but involves more criteria (10 neurological and 11 physical) than the Ballard system and hence is a longer assessment.

Although gestational age may be accurately determined, growth and development at each week of gestation can vary from fetus to fetus. In low- and middle-income countries, for example, more than one-quarter of live births involve newborns who are small for gestational age (SGA), because their birthweight or length was at or below the 10th percentile. SGA newborns may be at increased risk of learning disabilities and conditions such as attention deficit/hyperactivity disorder (ADHD).