Pregnancy Calculator

Track pregnancy week by week from LMP or conception date: trimester, due date, and milestone timeline.

Due Date From LMP — Naegele's Rule

BrainyCalculators editorial insight — unique to this tool

EDD = LMP + 280 days (40 weeks) under Naegele's rule — ultrasound dating in first trimester adjusts if LMP uncertain. Indian ANC schedules track trimester milestones; only ~4% deliver on exact EDD. Conception date ≈ LMP + 14 days for 28-day cycles.

When to use this calculator

Use for gestational age and due date from LMP. For ovulation timing before conception, use Ovulation.

Predicting fertile days before conception?

This page tracks gestational age after pregnancy starts. For ovulation timing, use the Ovulation Calculator →

First Day of Last Period (LMP)

Average Cycle Length (days)

What is a Pregnancy Calculator?

A pregnancy calculator converts last menstrual period or conception date into gestational age, trimester, and estimated due date using standard 40-week obstetric dating.

Use this page for ongoing week-by-week tracking after pregnancy is confirmed. Ovulation calculator predicts fertile window before conception.

Due date calculator focuses on EDD from LMP with clinical dating nuance.

How the Due Date Is Calculated

Due Date = LMP + 280 days (40 weeks)
Adjusted for cycle length: ± (cycle − 28) days

Naegele's rule adds 280 days (40 weeks) to the first day of the last menstrual period. Only a healthcare provider can confirm your due date.

How the Pregnancy Calculator Works

Formula, assumptions, and calculation steps for this health tool.

Methodology

Health calculators use published screening formulas and common planning rules to estimate body, nutrition, pregnancy, or fitness metrics from user inputs.

Calculation Steps

Enter the personal measurements requested by the tool.
Convert height, weight, age, dates, or activity inputs to standard units.
Apply the health or fitness formula for the selected metric.
Show the estimate with practical ranges or interpretation where available.

Assumptions and Limits

Results are educational estimates, not diagnosis or medical advice.
Individual factors such as medication, pregnancy, and medical history can change interpretation.
Consult a clinician for personal health decisions.

Reference basis: Common public-health and sports-science screening formulas.

Real-World Applications

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Maternity Leave Planning

Calculate the EDD to determine the earliest and latest date to start maternity leave — UK employees must give 15 weeks' notice and can start leave 11 weeks before EDD; knowing the precise EDD enables accurate HR leave planning.

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Booking Prenatal Care Appointments

Calculate gestational age to book the first-trimester dating scan (10–14 weeks), combined screening blood test, and booking midwife appointment — all of which have narrow gestational age windows for optimal accuracy.

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Folic Acid & Supplement Timing

Folic acid is most critical in the first 12 weeks of pregnancy (neural tube closure occurs at weeks 4–6). Knowing gestational age helps confirm whether supplementation started within the critical window or whether any gaps in early supplementation occurred.

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Travel & Flight Restrictions

Most airlines prohibit travel for single pregnancies beyond 36 weeks and twin pregnancies beyond 32 weeks. Calculate gestational age at the travel date to confirm compliance with airline medical policies before booking flights.

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Exercise & Activity Planning

Pregnancy fitness guidelines change by trimester — activities appropriate at 12 weeks (running, HIIT) may not be recommended at 32 weeks. Gestational age guides safe exercise selection and intensity modification throughout pregnancy.

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Baby Shower & Registry Timing

Tradition and practical preparation timelines suggest planning a baby shower at around 28–34 weeks — early enough to receive gifts before the birth but late enough to have detailed product preferences. Knowing the EDD sets the planning timeline.

Common Mistakes

Using conception date instead of first day of LMP

Gestational age is counted from the first day of the last menstrual period — not from the date of intercourse or conception. Conception occurs approximately 2 weeks after the LMP in a standard 28-day cycle. Using the conception date underestimates gestational age by approximately 14 days.

Not adjusting for irregular or long cycle lengths

The standard due date calculation assumes a 28-day cycle with ovulation on day 14. Women with longer cycles (e.g., 35 days) ovulate around day 21 — their EDD should be shifted 7 days later. Failure to adjust means the LMP-based date may overestimate gestational age.

Treating the EDD as a fixed birth date

The EDD is a statistical estimate — approximately 5% of babies are born on their exact due date. 80% of births occur within 2 weeks either side of the EDD. Fixating on the specific due date creates anxiety and may lead to pressure for unnecessary induction before term.

Not updating the EDD after a first-trimester ultrasound

ACOG guidelines recommend updating the EDD when first-trimester ultrasound dating differs by more than 7 days from the LMP-based estimate. Many women continue using the LMP date when a more accurate ultrasound-based EDD is available from their dating scan report.

Confusing weeks-and-days notation

Gestational age is expressed as weeks+days (e.g., "28+3" means 28 weeks and 3 days). "28 weeks pregnant" means you are in week 28 — you have completed 27 full weeks. This distinction affects the timing of gestational age-dependent clinical thresholds like the 24-week viability threshold.

Pregnancy Trimesters & Key Development Milestones

Trimester	Weeks	Key Developments
First	Weeks 1–12	Organogenesis; neural tube closes wk 4–6; heartbeat wk 6; miscarriage risk highest
Second	Weeks 13–26	Rapid growth; foetal movement felt (wk 16–22); anomaly scan wk 18–20
Third	Weeks 27–40	Lung maturation; foetal positioning; Braxton Hicks; GBS swab wk 36–37
Late preterm	Weeks 34–36+6	Viable but NICU admission likely; lung development continuing
Full term	Weeks 37–41+6	Optimal delivery window; monitoring increases at 41 weeks
Post-term	Week 42+	Induction typically offered; increased stillbirth risk monitoring

References

ACOG. "Methods for Estimating the Due Date." Committee Opinion No. 700. ACOG, 2017.
NICE. Antenatal Care Guideline NG201. National Institute for Health and Care Excellence, 2021.
WHO. WHO Recommendations on Antenatal Care for a Positive Pregnancy Experience. WHO, 2016.
Moore, K.L. et al. The Developing Human: Clinically Oriented Embryology. Elsevier, 2019.
Naegele, F.K. Lehrbuch der Geburtshülfe. Groos & Heyer, 1812.