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Gestational Age

By

Robert L. Stavis

, PhD, MD

Last full review/revision Jul 2019| Content last modified Jul 2019
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Gestational age and growth parameters help identify the risk of neonatal pathology. Gestational age is the primary determinant of organ maturity.

Gestational age is loosely defined as the number of weeks between the first day of the mother's last normal menstrual period and the day of delivery. More accurately, the gestational age is the difference between 14 days before the date of conception and the day of delivery. Gestational age is not 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

  • Date of conception

  • First day of the last menstrual period and cycle length

  • Fetal ultrasonography

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

The estimated date of confinement (EDC) is the date birth is expected (the due date). The EDC 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 and recorded contemporaneously, the menstrual history is relatively reliable.

Ultrasonographic measurements of the fetus in the 1st trimester give the most accurate estimate of gestational age when other information is lacking. When the date of conception is unknown and the menstrual cycles are irregular, unreliable, or information about them is not available, ultrasonography may be the sole source of the estimated date of confinement. In cases where the estimated date of confinement is calculated based on menstrual cycle data (see the American College of Obstetricians and Gynecologists' [ACOG] Methods for Estimating Due Date), the ACOG recommends using the ultrasonographic date if it differs from the calculated date by

  • > 5 days: Up to 8 6/7 weeks gestational age

  • > 7 days: For 9 to 13 6/7 weeks gestational age

Because ultrasonographic estimates are less accurate later in pregnancy, 2nd and 3rd trimester ultrasonographic results should rarely be used to revise those done during the 1st trimester.

Newborn physical examination findings also allow clinicians to estimate gestational age using the new Ballard score. 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 drugs (eg, magnesium sulfate given during labor). Because the Ballard score is accurate only within plus or minus 2 weeks, it should be used to assign gestational age only when there is no reliable obstetrical information about the estimated date of confinement or there is a major discrepancy between the obstetrically defined gestational age and the findings on physical examination.

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. The Journal of Pediatrics 119(3):417–423, 1991; used with permission of the CV Mosby Company.)

Assessment of gestational age—new Ballard score

Based on gestational age, neonates are classified as

  • Premature: < 34 weeks gestation

  • Late pre-term: 34 to < 37 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 and beyond

  • Postmature: > 42 weeks

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