How Babymetric Calculates

Every Babymetric calculation is a direct implementation of published medical formulas and guidelines. There are no external API calls — calculations run entirely in the user’s browser, and inputs never leave the device. The formulas, accuracy, and limits for each calculator are described below.

Due Date (Naegele’s Rule)

EDD = LMP + 280 days (= LMP − 3 months + 7 days)

The 19th-century standard for dating pregnancy, based on a 280-day average length. Assumes a 28-day cycle with ovulation 14 days after the LMP. About 5% of births occur on the predicted EDD, 65% within ±1 week, and 90% within ±2 weeks. For irregular cycles or IVF pregnancies, ultrasound-based dating (especially first-trimester CRL) is more accurate.

Ovulation and Fertile Window

Ovulation = next period start − 14 days; fertile window = ovulation −5 days to +1 day

Based on the observation that the luteal phase stays close to 14 days in most cycles. The fertile window is set as a 6-day span covering sperm survival (up to 5 days) and ovum survival (~24 hours). Accuracy drops with irregular ovulation (e.g., PCOS).

Fetal Weight (Hadlock-4)

log10(EFW) = 1.3596 − 0.00386·AC·FL + 0.0064·HC + 0.00061·BPD·AC + 0.0424·AC + 0.174·FL

Hadlock’s 1985 four-parameter regression. Inputs: head circumference (HC), biparietal diameter (BPD), abdominal circumference (AC), femur length (FL). Average error is roughly ±10%. Because ultrasound measurements themselves carry error, the output is an estimate.

Growth Percentile (WHO Standards)

Z = (X − M) / SD → cumulative normal → percentile

For ages 0–5 years, height/weight/head circumference is converted to a Z-score via the WHO LMS tables (2006) and mapped to a percentile through the cumulative normal distribution. Mean and SD are applied per sex and age. For ages over 5, country-specific charts apply (CDC for US, KSPS for Korea, MHLW for Japan).

hCG Doubling Time

T(double) = (t2 − t1) · ln(2) / ln(hCG2 / hCG1)

Estimates how long it takes hCG to double between two measurements. In early pregnancy (weeks 4–6), the normal range is 48–72 hours. Used together with ultrasound to flag abnormal patterns (ectopic pregnancy, threatened miscarriage), but is not diagnostic on its own.

Pregnancy Weight Gain (IOM/NAM 2009)

BMI category → total recommended range (singleton vs. twin)

IOM/NAM 2009 (adopted by ACOG) provides ranges by pre-pregnancy BMI for singletons across four categories (underweight, normal, overweight, obese). First trimester gain is 0.5–2 kg total; weeks 14–40 follow a per-week pace of ~0.35–0.5 kg for normal BMI. Twins follow a separate table; underweight-BMI twin data is sparse, so the underweight category falls back to normal-BMI values.

Common limits and disclaimer

All formulas are estimates based on population averages. Personal decisions need additional context — family history, lifestyle, comorbidities, direct ultrasound measurements, medication, and more. This site is not a diagnostic or therapeutic medical device. All results are reference estimates; medical decisions belong with your obstetrician, pediatrician, or other qualified clinician.