Gestational Age

Gestational age and growth parameters help identify the risk of neonatal pathology. Gestational age is the primary determinant of organ maturity.

Neonatal gestational age is usually defined by counting the number of weeks between the first day of the mother's last normal menstrual period and the date of delivery. However, determining gestational age based on the last menstrual period may be inaccurate if a pregnant patient has irregular menses. More accurately, the gestational age is the difference between 14 days before the date of conception and the day of delivery. Using the last menstrual period to determine gestational age does not determine the actual embryologic age of the fetus, but it is the universal standard among obstetricians and neonatologists for discussing fetal maturation.

Embryologic age is the time elapsed from the date of conception to the date of delivery and is 2 weeks less than the gestational age. Women may estimate the date of conception based on their time of ovulation as identified by in-home hormonal testing and/or basal body temperature measurements. However, the date of conception is definitively known only when in vitro fertilization or other assisted reproductive techniques are used.

Estimations of gestational age can be based on

• First day of the last menstrual period and cycle length

• Date of ovulation or conception

• Fetal ultrasonography

• Physical parameters after birth (eg, using the Ballard score)

The estimated date of delivery (EDD) is the date birth is expected (the due date). The EDD can be calculated as

• The date of conception + 266 days

• The last menstrual period (LMP) + 280 days (40 weeks) for women with regular, 28-day menstrual cycles

• The LMP + 280 days + (cycle length – 28 days) for women with regular menstrual cycles other than 28 days duration

When periods are regular, the menstrual history is a relatively reliable method of determining estimated date of delivery.

When other information is lacking, ultrasonographic measurements of the fetus in the first trimester give the most accurate estimate of gestational age. When the date of conception is unknown and menstrual cycles are irregular or information about them is not available, ultrasonography may be the sole source of the estimated date of delivery.

The gestational age based on the last menstrual period and based on the first fetal ultrasound in the current pregnancy are compared. If these age estimates are inconsistent, the estimated date of delivery (and, thus, the estimated gestational age) may be changed, depending on the number of weeks and the degree of inconsistency. The American College of Obstetricians and Gynecologists (ACOG) (see Methods for Estimating Due Date) recommends using the date based on ultrasonographic measurements if it differs from the menstrual date by

• At ≥ 8 6/7 weeks of gestation: > 5 days

• At 9 to 15 6/7 weeks of gestation: > 7 days

• At 16 to 21 6/7 weeks of gestation: > 10 days

• At 22 to 27 6/7 weeks of gestation: > 14 days

• At ≥ 28 weeks of gestation: > 21 days

Reconciling the menstrual and ultrasonographic dates is done only after the first ultrasound in the current pregnancy—estimated date of delivery is not changed based on subsequent ultrasounds. Because ultrasonographic estimates are less accurate later in pregnancy, second and third trimester ultrasonographic results should rarely be used to change estimated gestational age, and, if changing the estimated date of delivery is considered, a specialist in fetal ultrasonography should be consulted.

Newborn physical examination findings are also used by clinicians to estimate gestational age, using the new Ballard score. The Ballard score is accurate only within plus or minus 2 weeks. Newborn clinical assessments of gestational age have been found to overestimate gestational age in preterm infants and underestimate gestational age in small-for-gestational-age infants (1). Therefore, physical examination assessment of gestational age should be used to assign gestational age and for decisions regarding care only when there is no reliable obstetrical information about the estimated date of delivery or there is a major discrepancy between the obstetrically defined gestational age and the findings on physical examination.

The Ballard score is based on the neonate's physical and neuromuscular maturity and can be used up to 4 days after birth (in practice, the Ballard score is usually used in the first 24 hours). The neuromuscular components are more consistent over time because the physical components mature quickly after birth. However, the neuromuscular components can be affected by illness and medications (eg, magnesium sulfate given during labor).

Assessment of Gestational Age—New Ballard Score

Scores from neuromuscular and physical domains are added to obtain total score. (Adapted from Ballard JL, Khoury JC, Wedig K, et al: New Ballard score, expanded to include extremely premature infants. Pediatrics 119(3):417–423, 1991. doi: 10.1016/s0022-3476(05)82056-6; used with permission of the CV Mosby Company.)

Based on gestational age, neonates are classified as (2)

• Preterm: < 37 weeks of gestation

• Late preterm: 34 to 36 6/7 weeks

• Early term: 37 0/7 weeks through 38 6/7 weeks

• Full term: 39 0/7 weeks through 40 6/7 weeks

• Late term: 41 0/7 weeks through 41 6/7 weeks

• Postterm: ≥ 42 0/7 weeks