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Gestational Age


Arcangela Lattari Balest

, MD, University of Pittsburgh, School of Medicine

Reviewed/Revised Oct 2022
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Gestational age is usually defined as the number of weeks between the first day of the mother's last normal menstrual period and the date of delivery. More accurately, the gestational age is the difference between 14 days before the date of conception and the day of delivery. However, determining gestational age based on the last menstrual period may be inaccurate if the mother has irregular menses. 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

  • First day of the last menstrual period and cycle length

  • Date of 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 and recorded contemporaneously, the menstrual history is relatively reliable.

Ultrasonographic measurements of the fetus in the first trimester give the most accurate estimate of gestational age when other information is lacking or unreliable. 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 delivery. In cases where the estimated date of delivery 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

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

  • At 9 to 13 6/7 weeks gestation: > 7 days

Because ultrasonographic estimates are less accurate later in pregnancy, second and third trimester ultrasonographic results should rarely be used to revise those done during the first trimester.

Newborn physical examination findings are used by clinicians to estimate gestational age, using the new Ballard score Assessment of gestational age—new Ballard score Assessment of gestational age—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 General references Gestational age and growth parameters help identify the risk of neonatal pathology. Gestational age is the primary determinant of organ maturity. Gestational age is usually defined as the number... read more ). 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 drugs (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.)

Assessment of gestational age—new Ballard score

General references

  • 1. Lee AC, Panchal P, Folger L, et al: Diagnostic accuracy of neonatal assessment for gestational age determination: A systematic review. Pediatrics 140(6):e20171423, 2017. doi: 10.1542/peds.2017-1423

  • 2. Spong CY: Defining "term" pregnancy: Recommendations from the Defining "Term" Pregnancy Workgroup. JAMA 309(23):2445–2446, 2013. doi: 10.1001/jama.2013.6235

More Information

The following English-language resource may be useful. Please note that THE MANUAL is not responsible for the content of this resource.

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