Pregnancy week calculator — gestational age, trimester, due date

What does the pregnancy week calculator do?

Three calculators in one tool: current gestational age, projected due date and post-birth German well-baby visit windows. Gestational age is reported in the clinical W+D format (e.g. 23+4 for 23 completed weeks plus 4 days). The trimester is derived from the GA per the boundaries set by the American College of Obstetricians and Gynecologists (ACOG): T1 = weeks 1–13, T2 = 14–27, T3 = 28 to birth. The due date (estimated term, ET) is computed via the Naegele rule: first day of the last menstrual period (LMP) plus 280 days, adjusted by the deviation of the user’s cycle from 28 days.

Three input paths are supported because women anchor their pregnancy timeline differently. Some remember an LMP clearly; others know the exact conception date after an IVF cycle; still others get a measured gestational age at the first ultrasound. The calculator converts every anchor internally to LMP and works from there — so all three inputs yield identical outputs given equivalent information.

How does the gestational age count work?

In obstetrics, gestational age is counted from the first day of the last menstrual period, not from conception. This menstrual dating has two reasons: LMP is something the patient can remember, whereas the fertilization day is usually unknown; and standardization matters — a GA reading is only comparable across practices, hospitals and countries if everyone uses the same reference line. The WHO and ACOG converged on LMP-based counting, and it appears in essentially every prenatal record and pregnancy software worldwide.

Concretely: if today is day 70 since LMP, the GA reads 10+0 (= 10 completed weeks, 0 days remaining). If today is day 88, it reads 12+4. The + is a separator, not arithmetic addition. Clinical software like Astraia, Eviewer and country-specific maternity records all use this exact format.

Fertilization itself occurs roughly two weeks after LMP — for a 28-day cycle, typically on day 14 (ovulation). So GA week 4 corresponds to “about 2 weeks since conception”, GA week 6 to “4 weeks since conception”, and so on. This two-week offset confuses many first-time pregnancies — “I’m 8 weeks” feels like 8 weeks of embryo existence, but is actually closer to 6.

How does the Naegele rule work?

Franz Karl Naegele published the eponymous due-date estimate in Heidelberg in 1812: take the first day of the last menstrual period, subtract three months, add one year and seven days. Modern formulation: due date = LMP + 280 days. 280 days corresponds to 40 weeks, or roughly 9 months plus 7 days — the international standard for “average human gestation from LMP”.

The rule has two assumptions: a regular 28-day cycle, and ovulation on day 14. Neither assumption holds exactly for about half of all women. Longer cycles ovulate later (a 35-day cycle ovulates around day 21), shorter cycles earlier. The calculator corrects automatically — enter your average cycle length and the due date shifts by the cycle-deviation from 28.

Important caveat: Naegele yields an expected value, not a prediction. Statistically, only about 5 % of babies are born exactly on the projected due date. The “normal” delivery range spans GA 37+0 to 41+6 — anything earlier is preterm, anything later is post-term. A 2022 Cochrane review found the median delivery week for first-time mothers is around 40+5, for multiparas around 40+3. The due date is realistically an expected value in a distribution, not a precise calendar date.

When is early ultrasound more accurate than LMP math?

When the LMP is uncertain or the cycle is irregular, early ultrasound dating beats LMP-based calculation. The crucial advantage: between weeks 8 and 13, embryos grow very uniformly — variation in the crown-rump length (CRL) measurement is small. A measurement in this window achieves a GA accuracy of ±5 days. In the second trimester accuracy widens to ±10–14 days; in the third trimester it widens further to ±21 days, because larger fetuses individuate more in their growth trajectories.

ACOG Committee Opinion 700 (2017) states: if the LMP-based due date differs from a first-trimester ultrasound-based date by more than 7 days, the ultrasound date becomes the new reference. This “re-dating” is clinically important — an overestimated due date can trigger unnecessary inductions for perceived post-term pregnancy; an underestimated date can produce false growth-restriction alarms.

The calculator supports an ultrasound input mode specifically for this case: enter the scan date and the measured gestational age (weeks + days), and the tool back-calculates LMP, then forward-calculates the current GA and projected due date.

Which trimester boundaries are correct?

There are two common conventions:

• ACOG / US standard (used in this tool): T1 = weeks 1–13, T2 = 14–27, T3 = 28+. The most common convention in modern obstetric software worldwide.
• Classical European variant: T1 = weeks 1–12, T2 = 13–28, T3 = 29+. Older textbooks and some European maternity records still use this.

Both are “correct” — they differ by one week at the boundaries. Trimester labels are not used for rigid classification but as broad phase markers with distinct risk profiles: T1 has the highest miscarriage risk and contains organogenesis; T2 is usually the quietest and most energetic phase; T3 becomes physically demanding with birth preparation as the main theme. The one-week difference between conventions is clinically not relevant.

What are the German U-visits?

The U-visits are Germany’s mandated pediatric wellness checks. Their content is codified in the “Kinder-Richtlinie” issued by the Federal Joint Committee (G-BA) — the supreme body of statutory health insurance. Every U-stage has a fixed content and a legally defined time window. Statutory insurance covers the visit fully if it happens WITHIN the window; outside the window, the cost may revert to the parents.

• U1 right after birth: APGAR score, basic newborn screening, vital signs.
• U2 day 3–10 of life: physical exam, reflexes, metabolic screening (PKU and others).
• U3 week 4–5: first proper follow-up, weight/length/head circumference, abdominal ultrasound.
• U4 month 3–4: vaccination scheduling, motor-skill development.
• U5 month 6–7: vision and hearing testing, fine and gross motor skills.
• U6 month 10–12: language comprehension, standing/walking, social bonding.

The calculator shows U-visit windows, not exact appointment dates — the pediatric practice books actual slots after birth. Anchor calculations are useful for long-range planning (parental-leave blocks, grandparent visits, daycare applications) months ahead of the birth itself.

Which factors most affect the actual delivery week?

The strongest influences on real delivery timing are statistically robust:

1. Parity (which pregnancy this is): first-time mothers deliver about 2 days later in the median than multiparas.
2. Multiple pregnancy: twins deliver at a median GA 36+0, triplets at 32+0, quadruplets at 30+0.
3. Family history: if the mother herself was born after week 41, the child is 2.1× more likely to be a late arrival (Norwegian cohort, ~50,000 births).
4. Ancestry: modest differences in median pregnancy length — East-Asian women about 3 days earlier than Northern-European, Black women about 5 days earlier.
5. Maternal obesity: BMI > 30 extends median pregnancy by 1–2 days and raises post-term risk.

These influences are individually small — statistical scatter within any subgroup is far larger than between-subgroup means. The due date remains a ±2-week 1-sigma estimate.

How should I treat early pregnancy and the wait for confirmation?

Until the first ultrasound-confirmed GA (typically week 6–8 with detectable heartbeat), the GA calculation is purely an LMP projection — it does NOT confirm an intact pregnancy, intrauterine implantation location, or detectable heartbeat. A positive pregnancy test is enough for the arithmetic GA; clinical confirmation requires the early ultrasound.

In this phase Naegele is the only anchor. Women with irregular cycles often see a 1–2 week re-dating at their first ultrasound — that is normal and triggers a clinical “re-date” event. Practically: note the values from this calculator but accept correction at the first scan if the measured embryo size disagrees.

What methodology does this calculator use?

The calculator combines four standard sources into a consistent estimate:

1. Naegele rule (1812): due date = LMP + 280 days, cycle-adjusted.
2. ACOG Committee Opinion 700 (2017): trimester boundaries at weeks 1/14/28, early ultrasound re-dating when discrepancy exceeds 7 days.
3. G-BA Kinder-Richtlinie §4: U1–U6 windows relative to the projected birth.
4. Pure date math: all computation runs locally in JavaScript without network calls; UTC midnight is used as the stable day unit to avoid timezone shifts at day boundaries.

The calculation has no server component. Date input → GA computation → display all happens in the browser. No cookies are set, no third-party pixels are loaded, no analytics events are fired. A genuine privacy-first approach, not marketing theater.

What the calculator does NOT do

The calculator is a date utility, not medical diagnostics. It does NOT tell you whether the pregnancy is viable, whether complication risk is elevated, or whether any specific intervention is appropriate. This calculation is an informal estimate. It does not replace medical advice. For pregnancy questions — symptoms, unexplained pain, bleeding, reduced fetal movement in T3, suspected post-term status — consult your obstetrician or midwife. The GA value here is an orientation for scheduling and pregnancy context, not a clinical decision.

In particular, this tool does not provide guidance on medications, prenatal diagnostics, supplements or physical activity. Those topics belong in a personal conversation with your obstetrician or a qualified midwife who knows your history, your findings and your individual situation.