How is the due date calculated?

Naegele's Rule: add 280 days (40 weeks) to the first day of your Last Menstrual Period (LMP). Assumes 28-day cycle with ovulation on Day 14. First trimester ultrasound (8–12 weeks) is more accurate for irregular cycles.

What are the three trimesters?

First trimester: Weeks 1–13. All major organs form; highest miscarriage risk. Second trimester: Weeks 14–27. Most women feel best; baby movements felt at 18–20 weeks; anatomy scan at week 20. Third trimester: Weeks 28–40. Baby gains most weight; preparation for birth.

What is the difference between gestational age and fetal age?

Gestational age counts from LMP and is the standard medical measurement. Fetal (conceptional) age counts from conception — typically 2 weeks less than gestational age. All prenatal records use gestational age.

When should I see a doctor after a positive pregnancy test?

Most OBs schedule the first visit at 8–10 weeks. If you have a history of miscarriage, are over 35, or have chronic health conditions, request an earlier appointment at 6–7 weeks.

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Pregnancy Calculator

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Medically reviewed by Dr. P Nair, MBBS, MD (Community Medicine) · Author: Mayra

✓ Last reviewed: June 2026 · Methodology

Pregnancy due date is calculated using Naegele's Rule: add 280 days (40 weeks) to the first day of your last menstrual period (LMP). This assumes a 28-day cycle with ovulation on Day 14. For irregular cycles, adjust: due date = LMP + (280 − (28 − cycle length)) days. Ultrasound at 8–12 weeks is more accurate.

Due date and trimester timeline

Use this when: you need a quick, accurate result from pregnancy calculator without sign-up or tracking. All calculations run in your browser and no data is stored.

📜 ACOG source for pregnancy dating

Week calculations use Naegele’s rule (LMP + 280 days) with cycle-length adjustment, consistent with ACOG methods for estimating gestational age. First-trimester ultrasound remains the most accurate dating method — this tool is for orientation between appointments, not a substitute for clinical dating. Last reviewed: June 2026.

📊 Methodology: This calculator uses validated clinical formulas as cited in the sources below. Results are estimates for general informational purposes. Consult a qualified healthcare professional before making health decisions.

Last updated: May 2026

How Due Dates Are Calculated

The estimated due date is calculated using Naegele's rule: add 280 days (40 weeks) to the first day of your last menstrual period (LMP). If your average cycle is longer or shorter than 28 days, the calculator adjusts accordingly. Only about 5% of babies are born on their exact due date, but most arrive within two weeks of this estimate. Pairs well with the Due Date Calculator and the Ovulation Calculator.

Understanding Trimesters

Pregnancy is divided into three trimesters. The first trimester (weeks 1-12) is when major organ development occurs. The second trimester (weeks 13-26) is often considered the most comfortable period, when morning sickness typically subsides. The third trimester (weeks 27-40) involves rapid fetal growth and preparation for delivery.

Important Milestones

Key pregnancy milestones include: hearing the heartbeat at 6-7 weeks, feeling first movements (quickening) at 16-25 weeks, viability at about 24 weeks, and full term at 39-40 weeks. Your healthcare provider will schedule regular checkups and ultrasounds to monitor your pregnancy progress and baby's development.

Disclaimer

This calculator provides estimates for informational purposes only and should not replace professional medical advice. Always consult your healthcare provider for accurate dating and prenatal care guidance. Ultrasound dating, especially in the first trimester, is more accurate than LMP-based calculations.

Important health note

This calculator is for general information and self-checking only. It should not be used as a diagnosis, treatment plan, or replacement for advice from a qualified doctor, dietitian, trainer, or other healthcare professional. If you are pregnant, managing a medical condition, taking medication, recovering from illness, or making a major diet or exercise change, please speak with a professional before acting on the result.

How should you read and use the output?

The Pregnancy Calculator is built for people who want a fast answer without losing context. It keeps the calculation simple, shows the result clearly, and helps you understand what the number means before you use it in a real decision.

This calculator is designed to make a specific everyday calculation faster and clearer. It gives a structured result so you can compare options, check assumptions, or plan the next step with less manual work.

How pregnancy due date is calculated

The standard method for calculating a pregnancy due date is Naegele's Rule, developed in the early 19th century. It assumes a 28-day menstrual cycle with ovulation on day 14 and calculates from the first day of the last menstrual period (LMP):

Due Date = First day of LMP + 280 days (40 weeks)

Or equivalently: Add 7 days to the LMP, subtract 3 months, and add 1 year.

Example: LMP = 1 January 2026. Add 7 days → 8 January. Subtract 3 months → 8 October. Add 1 year → 8 October 2026. Due date: 8 October 2026

Only approximately 5% of babies are born on their exact due date. The normal delivery window is from 37 to 42 weeks of gestation. Delivery before 37 weeks is considered preterm; after 42 weeks is post-term.

The three trimesters — what happens each trimester

First trimester (Weeks 1–12): The most critical developmental period. All major organs and body systems begin forming. The embryo becomes a fetus by week 10. Common symptoms include morning sickness (nausea and vomiting, often throughout the day), fatigue, breast tenderness, frequent urination, and food aversions. The risk of miscarriage is highest in the first trimester — approximately 80% of miscarriages occur before 12 weeks.

Second trimester (Weeks 13–26): Often called the "golden trimester" — nausea usually subsides, energy returns, and the pregnancy becomes visually apparent. The baby's movements (quickening) are typically felt for the first time between weeks 18–22. This trimester includes the anatomy scan ultrasound (usually at 18–20 weeks) which checks fetal development and can reveal the baby's sex.

Third trimester (Weeks 27–40): The baby grows rapidly — gaining approximately 50% of its birth weight in the final trimester. Common symptoms include back pain, heartburn, difficulty sleeping, and Braxton Hicks contractions. The baby typically assumes the head-down position (cephalic presentation) in preparation for delivery between weeks 32–36.

What happens in each trimester of pregnancy week by week?

Week 6: Heartbeat detectable by transvaginal ultrasound
Week 8: All major organs have begun to form; embryo is now called a fetus
Week 12: End of first trimester; risk of miscarriage drops significantly; nuchal translucency scan
Week 16–18: Amniocentesis window if indicated; baby's sex may be visible on ultrasound
Week 20: Anatomy scan; halfway point
Week 24: Viability milestone — babies born at 24+ weeks can survive with intensive care
Week 28: Third trimester begins; glucose tolerance test for gestational diabetes
Week 37: "Full term" — baby is considered ready for birth
Week 40: Estimated due date

Gestational age vs fetal age

Gestational age counts from the first day of the last menstrual period — 2 weeks before conception typically occurs. Fetal age (also called embryonic age) counts from the moment of fertilisation. A baby at "40 weeks gestational age" is actually approximately 38 weeks of fetal development. Medical professionals universally use gestational age because the LMP date is usually known, while the exact conception date typically is not.

Prenatal care schedule (India — standard)

First trimester: Booking appointment (8–12 weeks), blood tests, ultrasound for dating and nuchal translucency screening
Second trimester: Anomaly scan at 18–20 weeks; regular weight, blood pressure, and urine checks every 4 weeks
Third trimester: Visits every 2 weeks from week 28, then weekly from week 36; fetal position assessment; birth plan discussion

Frequently asked questions about pregnancy

How accurate is the due date calculation? Naegele's Rule assumes a 28-day cycle and ovulation on day 14. Women with irregular cycles or cycles longer/shorter than 28 days may have different ovulation timing. Early ultrasound (8–12 weeks) provides the most accurate due date estimate by measuring fetal size.

What if I don't know my last menstrual period date? An early ultrasound can estimate gestational age and calculate an approximate due date from fetal measurements. This is common for women with irregular periods or those who conceive shortly after stopping hormonal contraception.

What is a full-term pregnancy? Full term is defined as 39 weeks 0 days to 40 weeks 6 days. Early term is 37–38 weeks 6 days. Late term is 41 weeks 0 days to 41 weeks 6 days. Post-term is 42 weeks and beyond. Babies born at different term stages have different average outcomes.

Sources & References

ACOG — Calculating Gestational Age — Naegele's rule: LMP + 280 days; cycle-length correction; ultrasound supersedes LMP dating after 12 weeks
WHO — Antenatal Care Guidelines — WHO recommended gestational-age dating and trimester milestones

Pregnancy milestones by trimester

Week	Trimester	Key Milestone
4–5	First	Positive home pregnancy test; embryo implants
6–8	First	Heartbeat detectable on ultrasound
10–13	First	First trimester screening; nuchal translucency scan
13	First ends	Risk of miscarriage drops significantly
16–18	Second	Gender detectable; quadruple screen blood test
18–22	Second	Anatomy ultrasound (anomaly scan)
20	Second	Halfway point; most women feel best
24–28	Second	Glucose tolerance test; baby viable if premature
28	Third begins	Fetal movement monitoring begins
32–34	Third	Baby in head-down position (usually)
36	Third	Baby considered early term; GBS test
37–40	Third	Full term; birth expected any time
40+	Post-term	Induction discussed if no labour by 41–42 weeks

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⚕️ Medical disclaimer: This calculator provides educational estimates only and is not a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider about your specific situation. Formulas and reference ranges follow WHO and ICMR published guidelines — see our methodology.

Health disclaimer: This calculator provides estimates for informational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Results are based on population averages and may not apply to individuals with specific health conditions. Always consult a qualified healthcare professional. Reviewed by Dr. P Nair, MBBS, MD.