How it works
Each measurement is converted to a gestational age with the selected chart, then the calculator decides which ones to trust, following ISUOG, SOGC and ACOG guidance:
- Before 14 weeks (CRL ≤ 84 mm): CRL alone. It is the most accurate dating measurement of the whole pregnancy (95% range about ± 5–8% of the age, i.e. ± 3–6 days). Other measurements entered at this stage are shown for comparison but not used.
- CRL over 84 mm: fetal flexion makes CRL unreliable, so it is dropped in favour of the head and long-bone measurements.
- 14 weeks onward: a combination of parameters is more accurate than any one of them. With the Hadlock chart the estimate is the mean of the individual BPD, HC, AC and FL ages (Hadlock showed averaging is as good as his multi-parameter regressions); with INTERGROWTH-21st it is the HC + FL equation (or HC alone). HC is the most robust single parameter; BPD depends on head shape and AC on fetal nutrition.
- Discordant measurements: if one parameter disagrees with the others by more than its own 95% range it is excluded from the mean and flagged. A short femur, a small abdomen or an odd-shaped head are findings about the fetus, not about its age. You can override any inclusion with the tick boxes.
- Accuracy falls with gestation: roughly ± 1 week at 12–18 weeks, ± 1.5 weeks at 18–24, ± 2 weeks at 24–30 and ± 3 weeks after 30 weeks (Hadlock 1984). Third-trimester dating should only set the due date when nothing earlier exists, ideally confirmed by a growth scan 2–3 weeks later (SOGC).
- Established dates are never revised from a later scan. On the Prior report tab, type the biometric age the scanner reports today under Current biometry (or, if the machine has not calculated it, enter the raw measurements on the Biometry tab) and it is compared with the extrapolated age and judged against the ACOG Committee Opinion 700 thresholds (> 5 days up to 8+6, > 7 days 9+0–15+6, > 10 days 16+0–21+6, > 14 days 22+0–27+6, > 21 days from 28+0). A discrepancy beyond those limits from an earlier-dated pregnancy suggests abnormal growth or measurement error rather than a dating problem.
Equations
| Chart | Measurement | Gestational age | 95% range |
|---|---|---|---|
| Hadlock 1992 | CRL (cm) | weeks = e1.684969 + 0.315646·CRL − 0.049306·CRL² + 0.004057·CRL³ − 0.000120456·CRL⁴ | ± 8% of the age |
| Hadlock 1984 | BPD (cm) | weeks = 9.54 + 1.482·BPD + 0.1676·BPD² | ± 1.19 / 1.73 / 2.18 / 3.08 / 3.20 wk |
| HC (cm) | weeks = 8.96 + 0.540·HC + 0.0003·HC³ | ± 1.19 / 1.48 / 2.06 / 2.98 / 3.20 wk | |
| AC (cm) | weeks = 8.14 + 0.753·AC + 0.0036·AC² | ± 1.66 / 2.06 / 2.18 / 2.96 / 3.04 wk | |
| FL (cm) | weeks = 10.35 + 2.460·FL + 0.170·FL² | ± 1.38 / 1.80 / 2.08 / 2.96 / 3.12 wk | |
| INTERGROWTH-21st 2014 | CRL (mm) | days = 40.9041 + 3.21585·√CRL + 0.348956·CRL | SD = 2.39102 + 0.0193474·CRL days |
| INTERGROWTH-21st 2016 | HC, FL (mm) | days = e0.03243·(ln HC)² + 0.001644·FL·ln HC + 3.813 | SD = 0.04009·GA − 1.149 days |
| HC (mm) | days = e0.05970·(ln HC)² + 0.000000006409·HC³ + 3.3258 | SD = 0.6492·(GA/100)³ + 2.991 days |
Hadlock 1984 ranges are ± 2 SD for the 12–18, 18–24, 24–30, 30–36 and 36–42 week bands; the ranges for means of several parameters (e.g. ± 7.6, 9.8, 12.6 and 17.1 days for all four) are from the same paper. INTERGROWTH-21st 95% ranges are ± 1.96 SD. The CRL equations were derived from 5–18 weeks (Hadlock) and 9+0–13+6 weeks (INTERGROWTH-21st); the late-pregnancy INTERGROWTH-21st equations from 14 to 34 weeks. Enter measurements in mm as your scanner reports them; the calculator converts to cm where the equation needs it. EDD is the date the pregnancy reaches 40+0.
Gestational ages (on the prior report, or the current biometry) can be typed as 22+3, 22w3d, 22 3, 22/3 or 22.3; a single number is whole weeks.
References
- Hadlock FP, Deter RL, Harrist RB, Park SK. Estimating fetal age: computer-assisted analysis of multiple fetal growth parameters. Radiology. 1984;152(2):497–501. doi:10.1148/radiology.152.2.6739822
- Hadlock FP, Shah YP, Kanon DJ, Lindsey JV. Fetal crown-rump length: reevaluation of relation to menstrual age (5–18 weeks) with high-resolution real-time US. Radiology. 1992;182(2):501–505. doi:10.1148/radiology.182.2.1732970
- Papageorghiou AT, Kennedy SH, Salomon LJ, et al., for the INTERGROWTH-21st Project. International standards for early fetal size and pregnancy dating based on ultrasound measurement of crown-rump length in the first trimester of pregnancy. Ultrasound Obstet Gynecol. 2014;44(6):641–648. doi:10.1002/uog.13448 · free full text
- Papageorghiou AT, Kemp B, Stones W, et al., for the INTERGROWTH-21st Project. Ultrasound-based gestational-age estimation in late pregnancy. Ultrasound Obstet Gynecol. 2016;48(6):719–726. doi:10.1002/uog.15894 · free full text
- Salomon LJ, Alfirevic Z, Da Silva Costa F, et al. ISUOG Practice Guidelines: ultrasound assessment of fetal biometry and growth. Ultrasound Obstet Gynecol. 2019;53(6):715–723. doi:10.1002/uog.20272 · free full text
- Committee on Obstetric Practice, American Institute of Ultrasound in Medicine, Society for Maternal–Fetal Medicine. Committee Opinion No. 700: Methods for Estimating the Due Date. Obstet Gynecol. 2017;129(5):e150–e154. doi:10.1097/AOG.0000000000002046 · free full text
- Butt K, Lim KI. Guideline No. 388: Determination of Gestational Age by Ultrasound. J Obstet Gynaecol Can. 2019;41(10):1497–1507. doi:10.1016/j.jogc.2019.04.010