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Gestational Age

By Robert L. Stavis, PhD, MD, Clinical Director, Neonatal ICUs, Main Line Health, Bryn Mawr, PA

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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 the time elapsed since the beginning of the woman's last menstrual period; it is usually counted in weeks and days. Gestational age is not the actual embryologic age of the fetus.

Embryologic age is the time elapsed since the date of conception. By convention (and assuming a 28-day menstrual cycle), the embryologic age is 2 wk less than the gestational age. Women may more accurately 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 because these techniques establish the date of conception.

Estimations of gestational age can be based on

  • Menstrual periods

  • Date of conception

  • 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 wk) 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 embryologic age. When menstrual cycles are irregular or data are unreliable or not available, ultrasonography may be the sole source of the EDC. If the EDC estimated by dates differs from the EDC estimated by ultrasonography, the American College of Obstetricians and Gynecologists (ACOG) recommends (see Methods for Estimating Due Date) using the ultrasonographic date if it differs from the calculated date by

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

  • > 7 days: For 9 to 13 6/7 wk 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 h). 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 wk, it should be used to assign gestational age only when there is no reliable obstetrical information about the EDC 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.)

Based on gestational age, each neonate is classified as

  • Premature: < 34 wk gestation

  • Late pre-term: 34 to < 37 wk

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

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

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

  • Postterm: 42 0/7 wk and beyond

  • Postmature: > 42 wk

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