Pregnancy Week Calculator — 40-Week Ladder with Fetal Milestones, Lengths & NICU Survival Curves

A Brief History of Pregnancy Staging

In 2026, a midwife at the John Radcliffe Hospital in Oxford books a new patient who is sure she is "four months pregnant". The midwife reaches for this calculator: LMP March 4 2026, today May 27 2026, total 84 days = 12w0d - actually just at the end of trimester 1, four weeks short of the "four months" the patient described. That mismatch is the entire reason gestational age is counted in weeks and days, not calendar months.

Counting pregnancy by week dates to Franz Karl Naegele's 1812 work in Heidelberg, which established the 40-week / 280-day standard. Lunar months (28 days each) had been used since antiquity, but the seven-day week gave finer resolution. By the mid-19th century the Royal Maternity Hospital in Edinburgh was recording gestational age in weeks routinely. The convention of counting from LMP (not conception) survives because most patients can recall the start of their period but not the moment of conception.

Ian Donald's 1958 Glasgow ultrasound work made it possible to compare gestational age (LMP-based) with embryonic / fetal size for the first time. By the 1970s sonographic charts of biparietal diameter (BPD) and crown-rump length (CRL) were published — Hadlock's 1985 and 1991 papers became the standard reference. The 2017 Intergrowth-21st Project at Oxford published a global reference set of 13 cohorts in 8 countries, confirming Hadlock's 50th-centile values hold across populations.

The trimester concept (Latin tri = three, mensis = month) entered obstetric writing in the early 20th century. The 12 / 27 / 40 boundaries were not standardised globally until the WHO Reproductive Health Library codified them in 2003. ACOG's Committee Opinion 579 (May 2013, reaffirmed 2024) added the early-term / full-term / late-term / post-term sub-divisions because elective delivery at 37-38 weeks was shown to triple the risk of NICU admission compared with 39-40 weeks.

The viability threshold has shifted over five decades thanks to neonatal medicine. In 1965 babies born before 28 weeks rarely survived. The introduction of antenatal corticosteroids (Liggins and Howie 1972), surfactant replacement therapy (Fujiwara 1980), and high-frequency ventilation (Bunnell 1986) progressively lowered the threshold. By 2026 the Vermont Oxford Network data shows 70% survival at 24w and 30% at 22w (the new periviability boundary per ACOG OCC 6, 2017).

Each milestone on the ladder above maps to a specific clinical event. The 11w-13w6d nuchal-translucency window was defined by Kypros Nicolaides at King's College London (1992) and now powers combined first- trimester screening with PAPP-A and free βhCG, sensitivity ~90% for trisomy 21. The 18w-22w anomaly scan follows the ISUOG 2019 protocol — standardised 50+ views of brain, face, heart, spine, abdomen, limbs and placenta. The 28-week GTT was added to UK NICE guidance in 2008 and to ACOG's in 2018.

For dating from a known LMP, the Due Date Calculator gives the EDD directly. For finding the conception date from an EDD, use Conception Date.