Pregnancy Due Date Calculator

Frequently Asked Questions

How accurate is the due date?

The estimated due date is a statistical midpoint. Only about 4-5% of babies arrive exactly on that day. Most births (70%) occur within 10 days before or after the date. First-trimester ultrasound dating is more accurate than LMP calculation and is used to adjust the due date in clinical practice when the two differ by more than 5-7 days.

What counts as full term?

Weeks 39 to 40 are full term. Week 37 to 38 is early term, week 41 is late term, and week 42 or beyond is post-term. The risk of complications increases slightly at both extremes. Most obstetric providers offer induction by week 41-42 for pregnancies that have not delivered spontaneously.

What is the difference between gestational age and fetal age?

Gestational age is counted from the LMP and is the standard used in obstetric care. Fetal age (conceptional age) is counted from conception, which typically occurs about 2 weeks after the LMP. Gestational age is approximately 2 weeks more than fetal age. All weeks quoted in prenatal care use gestational age.

My ultrasound due date is different from this calculator - which is right?

The ultrasound date should take precedence. First-trimester CRL measurement is the most accurate method of pregnancy dating, with a margin of error of about plus or minus 3-5 days. If your ultrasound estimate and LMP-based date differ by more than 5-7 days, your obstetric provider will typically adjust the official due date to the ultrasound estimate.

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General information only. Not medical advice.

Health & Medical Disclaimer: General information only. Not medical advice.

This calculator provides general health information only and is not medical advice. Results do not replace professional medical evaluation or diagnosis. Consult a licensed healthcare provider before making health decisions. Always seek immediate medical attention for emergencies.

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How This Calculator Works

Choose your starting point: the first day of your last menstrual period (LMP) or your conception date. From the LMP, the calculator adds 280 days (40 weeks) to set the estimated due date using Naegele's rule. If you enter a conception date instead, it subtracts 14 days to approximate the LMP, then adds 280 days. Gestational age is the number of days since the LMP divided by 7 (weeks), with the remainder as days. The trimester is: 1st trimester through week 12, 2nd trimester from week 13 through week 26, and 3rd trimester from week 27 onward. This calculator is for informational purposes only and does not constitute medical advice. Consult a qualified healthcare provider before making any health-related decisions.

Naegele's Rule Explained

Naegele's rule, proposed by German obstetrician Franz Karl Naegele in 1812, estimates the due date as LMP minus 3 months plus 7 days plus 1 year - which is mathematically equivalent to adding 280 days or 40 weeks. It assumes a 28-day cycle with ovulation on day 14. For women with longer or shorter cycles, the actual due date may differ. An equivalent manual calculation: take the first day of your last period, add 7 days, subtract 3 months from the month, and add 1 year. A due date estimated from ultrasound crown-rump length (CRL) in the first trimester is generally more accurate than LMP-based calculation and will override it in clinical practice if the two differ by more than 5 to 7 days.

Trimester Milestones

First trimester (weeks 1-12): Major organ development occurs. The embryo becomes a fetus around week 10. Morning sickness is most common in weeks 6-12. First trimester screening for chromosomal abnormalities (nuchal translucency ultrasound, cell-free DNA testing) typically occurs between weeks 10 and 13.

Second trimester (weeks 13-26): Generally the most comfortable trimester. Fetal movement (quickening) is often first felt around weeks 16-20. The anatomy ultrasound (20-week scan) screens for structural abnormalities. Glucose tolerance screening for gestational diabetes typically occurs around week 24-28.

Third trimester (weeks 27-40+): Rapid fetal weight gain and lung maturation. Group B Streptococcus screening around week 36. Fetal positioning matters for delivery. Full term is 39-40 weeks; early term is 37-38 weeks; late and post-term is 41-42 weeks.

Accuracy of the Due Date

The estimated due date is a statistical midpoint, not a precise prediction. Only about 4-5% of babies are born exactly on the due date. Roughly 70% of births occur within 10 days on either side. Spontaneous labor between weeks 39 and 41 is the most common range. Approximately 10% of pregnancies are delivered before week 37 (preterm), and about 10% go past week 41 (post-term, often prompting induction). First-trimester ultrasound using crown-rump length is the most accurate dating method when LMP is uncertain, irregular, or if the LMP-based date differs significantly from the ultrasound estimate.

IVF and Assisted Reproduction Dating

For IVF pregnancies, gestational age is typically calculated from the egg retrieval date plus 14 days (to approximate the equivalent LMP), then adding 266 days (38 weeks) from that approximated LMP. For frozen embryo transfers, the transfer date plus the number of days of embryo development (e.g., 5 days for a blastocyst) approximates the conception date, and the same 14-day offset to the estimated LMP is applied. Your fertility clinic will provide an estimated due date based on your specific cycle parameters, which is more precise than a calendar-based calculation from LMP.

Pregnancy Due Date Calculator

Frequently Asked Questions

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