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Pregnancy · Due date

Pregnancy Due Date & Trimester Estimator Calculator

Naegele’s rule + cycle-length adjustment (ACOG 2017).

Your details

I know my
First day of last menstrual period
Conception date

This is an estimate. First-trimester ultrasound dating remains the clinical gold standard when LMP is uncertain or cycles are irregular. Only a clinician confirms a pregnancy or a due date.

Current gestational age
0weeks
< 14 weeks≥ 41 weeks
First trimester (0-13 weeks)

The first trimester runs from 0 weeks (day of LMP) to 13 weeks 6 days. Early antenatal care matters — the first ultrasound is typically between 8-14 weeks and confirms dating.

Estimated due date
Fri, 07 May 2027
in 280 days
Gestational age today
0w 0d
First trimester

Naegele\u2019s rule (ACOG 2017). Ultrasound dating in the first trimester remains the clinical gold standard. This tool does not replace antenatal care.

Naegele’s rule, briefly

Franz Naegele (1830) observed that most pregnancies delivered 280 days (40 weeks) after the first day of the last menstrual period. The rule assumes a 28-day cycle with ovulation on day 14; longer cycles push the due date later by the extra days. Ultrasound dating in the first trimester is more accurate when cycles are irregular or LMP is uncertain.

When the scan overrides the LMP

ACOG 2017 recommends using first-trimester ultrasound dating (crown-rump length between 8 and 14 weeks) as the reference if it differs from LMP-based dating by more than 5-7 days. Later scans are less accurate for dating.

Foetal movement matters

After 24 weeks, awareness of your baby’s movement patterns is one of the most important self-monitoring habits. If you notice a persistent change or reduction in movements, contact your maternity team promptly — do not wait until the next appointment.

A due date is not a deadline

Only about 4-5% of pregnancies deliver exactly on the estimated date. Most deliver between 37 and 42 weeks. Your maternity team monitors closely after 41 weeks and usually discusses induction options between 41-42 weeks.

Category reference

CategoryRange
First trimester0 – 13w 6d
Second trimester14w 0d – 27w 6d
Third trimester28w 0d – delivery
At or past 40 weeks40+ weeks

Limitations & caveats

  • LMP-based dating is less reliable with irregular cycles, PCOS, recent hormonal contraception, or lactational amenorrhoea.
  • First-trimester ultrasound remains the clinical gold standard.
  • This tool does not replace antenatal care.
FAQ

Frequently asked questions

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Clinical Interpretation

First trimester (0-13 weeks)Normal

The first trimester runs from 0 weeks (day of LMP) to 13 weeks 6 days. Early antenatal care matters — the first ultrasound is typically between 8-14 weeks and confirms dating.

Common causes
  • •Early pregnancy — organogenesis phase
Lifestyle considerations
  • •Start (or continue) a prenatal vitamin with 400 µg folic acid daily — ideally began 3 months pre-conception
  • •Avoid alcohol, smoking, and unpasteurised foods
  • •Keep gentle physical activity if you were active pre-pregnancy — ACOG endorses continuing exercise unless a clinician says otherwise
  • •Register with an antenatal care provider early
When to consult a healthcare professional

Book your first antenatal appointment as soon as possible. Report any bleeding, severe pain, or persistent vomiting.

Medical Evidence

How we ground the Pregnancy Due Date & Trimester Estimator

Grade A · Clinical guidelines & meta-analysesLast reviewed Feb 12, 2026
Named clinical guidelines
  • ACOG Committee Opinion 700 (2017, reaffirmed 2022) — Methods for Estimating the Due Date
  • WHO — Recommendations on Antenatal Care for a Positive Pregnancy Experience (2016)
  • NICE NG201 — Antenatal Care (2021, updated 2023)
References

The Evidence Foundation

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