Gestational Age

Gestational age is the time period in which the fetus grows inside the uterus. Measured in weeks, gestational age has implications for the fetus's growth, as well as 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 formation of bones and cartilage. By the gestational age of Weeks 9 to 13, the genitalia have formed and the entire fetus weighs about 1 oz. By Weeks 21 to 23, the fetus's eyes have developed and the fetal heartbeat can be heard by stethoscope.

A normal pregnancy has a gestational age ranging from 38 to 42 weeks with a full-term pregnancy considered to be 40 weeks. Infants born at a gestational age of less than 38 weeks are considered premature and are susceptible to increased risks of morbidity [Page 436]and mortality. For example, the vast majority of infants born at 24 weeks will experience respiratory distress syndrome as the air sacs in the lungs have just begun forming.

Gestational age can be calculated before and after birth. Before birth, gestational age is calculated as the time from the first day of a pregnant woman's last menstrual cycle to the current date. Although often used to determine gestational age, health professionals recognize the potential for inaccuracies using this method due to variations in ovulation dates. Therefore, there are a number of other methods employed to accurately determine gestational age. One such method is the use of an ultrasound whereby growth can be determined through measurements of the head and abdomen. Another method is to determine the date of conception as per the mother's knowledge.

After birth, a newborn's gestational age can be measured using the Ballard Scale or the Dubowitz Exam. The Ballard Scale involves an examination of the neuromuscular and physical maturity of the newborn, while the Dubowitz Exam focuses on the neurological maturity of the newborn.

It is important to note that although gestational age may be accurately determined by the methods mentioned above, developmental growth at each week may vary from fetus to fetus. An estimated 3% to 10% of all newborns are determined to be small for gestational age (SGA) because their birthweight or length was determined to be less than the 3rd percentile. SGA newborns have higher incidence of learning disabilities, autism, and attention deficit disorder (ADD).