How to Calculate Gestational Age (GA)

Gestational age is expressed in completed weeks (and days), counted from the first day of the last menstrual period (LMP) - not from conception. A full-term pregnancy is 40 weeks (280 days). There are several methods, each with different accuracy and clinical contexts.

1. Naegele's Rule (LMP Method)

The most basic and widely used antenatal calculation. You need to know the first day of the LMP.

Formula:

Estimated Due Date (EDD) = LMP date − 3 months + 7 days

Example:

LMP = January 15, 2026
Subtract 3 months → October 15, 2025
Add 7 days → October 22, 2025 (EDD)

To find the current gestational age, count the number of completed weeks from the LMP to today.

Example:

LMP = January 1, 2026
Today = July 12, 2026
Days elapsed = 192 days
Gestational age = 27 weeks and 3 days

Limitations: Assumes a regular 28-day cycle with ovulation on Day 14. Unreliable if cycles are irregular, if the patient recently stopped oral contraceptives, or if she had spotting at implantation (which can be mistaken for a period). - The Developing Human: Clinically Oriented Embryology

2. Ultrasound Dating (Most Accurate)

Ultrasound is the gold standard when LMP is uncertain. The measurement used depends on the trimester. - Harriet Lane Handbook, 23rd ed.

Gestational Window	Best Measurement	Accuracy
6-10 weeks	Gestational sac (GS) + Crown-Rump Length (CRL)	±3-4 days
11-14 weeks	CRL (most accurate), BPD, FL	±3-4 days
15-28 weeks	BPD (most accurate), HC, FL, AC	±10 days
After 26-28 weeks	BPD + CI, HC, FL, AC (average)	±3 weeks

Key measurements:

CRL (Crown-Rump Length): Distance from the top of the fetal head to the bottom of the torso. Best measure in first trimester.
BPD (Biparietal Diameter): Width of the fetal head across the temples.
HC (Head Circumference)
FL (Femur Length)
AC (Abdominal Circumference): Least accurate for dating, better for growth.

In the 2nd and 3rd trimesters, use the average of BPD, HC, FL, and AC. Discard any measurement significantly discordant from the others. - Pfenninger & Fowler's Procedures for Primary Care

"Best obstetric estimate": Many countries now combine LMP and ultrasound, prioritizing first-trimester ultrasound when there is a discrepancy of >5-7 days between the two. - Park's Textbook of Preventive and Social Medicine

3. Fundal Height (Clinical/Low-Resource Settings)

Used when ultrasound is unavailable. In a singleton pregnancy:

Gestational age (weeks) ≈ Fundal height (cm) measured from the symphysis pubis to the top of the uterus

This is reliable between roughly 20-36 weeks, with ±2-3 week accuracy. After 36 weeks the head engages, and fundal height may decrease.

4. Ballard Score (Postnatal Estimation)

When obstetric dating is unavailable after birth, the New Ballard Score estimates gestational age from neuromuscular and physical maturity signs. It is most accurate at approximately 24 hours of life. - Harriet Lane Handbook, 23rd ed.

Neuromuscular maturity (6 signs, score -1 to 5 each):

Ballard Score Neuromuscular Maturity - scoring posture, square window, arm recoil, popliteal angle, scarf sign, heel-to-ear

Sign	What you assess
Posture	Degree of limb flexion at rest
Square window	Wrist flexion angle
Arm recoil	Elbow flexion angle after extending arms
Popliteal angle	Degree of knee extension
Scarf sign	How far the elbow crosses midline
Heel to ear	How close the foot reaches the head

Physical maturity (6 signs): Skin texture, lanugo, plantar creases, breast bud, ear cartilage, and genitalia.

Scoring: Add total neuromuscular + physical maturity scores, then use the Ballard conversion chart to read off the gestational age in weeks (score of 5 ≈ 22 weeks; score of 50 ≈ 44 weeks).

5. Postmenstrual Age (PMA)

Used in neonatal intensive care settings:

PMA = Gestational age at birth + Chronological age (weeks since birth)

Used until 2 years of age to track corrected development for premature infants.

Summary Table

Method	Setting	Accuracy
LMP (Naegele's rule)	Any (regular cycles)	±2 weeks
1st trimester US (CRL)	Antenatal	±3-4 days
2nd trimester US (BPD/HC/FL)	Antenatal	±10 days
3rd trimester US	Antenatal	±3 weeks
Fundal height	Low-resource, field	±2-3 weeks
Ballard score	Postnatal (NICU)	±2 weeks

Quick tip for exam questions: GA is always counted from the LMP (not conception). Embryonic/fertilization age is roughly 2 weeks less than gestational age - this distinction matters in embryology and in vitro fertilization. - The Developing Human: Clinically Oriented Embryology

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