Pregnancy Due Date Calculator

How Due Dates Are Calculated

Your estimated due date (EDD) is the date that marks the end of a full-term pregnancy — 40 weeks, or 280 days, from the first day of your last menstrual period (LMP). This is an estimate based on statistical averages, not a precise prediction of when your baby will arrive.

There are two main reference points clinicians use. The most common is LMP + 280 days (Naegele's rule), which assumes ovulation occurs on day 14 of a 28-day cycle. The second is conception date + 266 days, which counts from fertilisation rather than the last period. When an early ultrasound is available, providers may use the embryo's measured size to assign a gestational age and calculate the EDD from there.

This calculator combines all three methods and adjusts the LMP-based estimate for your cycle length, so the result is as accurate as a self-reported calculation can be. Always confirm your due date with your OB-GYN or midwife.

Why Only 5% of Babies Are Born on Their Due Date

The due date is really the 50th-percentile estimated delivery date — the midpoint of a range, not a fixed day. Research shows that most births cluster between 38 and 42 weeks from LMP, and only about 5% of babies are born on their exact calculated due date.

Normal biological variation in pregnancy length is large. Factors including genetics, the baby's sex, whether it's a first pregnancy, and even the mother's own birth gestational age all influence how long a pregnancy lasts. Your baby can be perfectly healthy and full-term whether born a week before or a week after your EDD. The due date is a planning target, not an expiry date.

Naegele's Rule Explained

Naegele's rule is the standard formula for calculating a due date from LMP. It was described by German obstetrician Franz Karl Naegele in the early 19th century and remains the basis for due-date estimation worldwide.

The formula can be expressed two equivalent ways:

• Add 280 days to the first day of the LMP.
• Add one year, subtract three months, and add seven days to the LMP date.

Both produce the same result. The rule assumes a 28-day cycle with ovulation on day 14. If your cycle length differs from 28 days, this calculator adjusts the EDD by the difference — for example, a 35-day cycle shifts the due date approximately 7 days later than the standard calculation. This makes the estimate meaningfully more accurate for people with longer or shorter cycles.

Pregnancy Trimesters Breakdown

Pregnancy is conventionally divided into three trimesters based on gestational age from LMP:

• First trimester (weeks 1–13): The period of most rapid development. Major organs begin to form, the heartbeat is detectable around week 6, and the embryo becomes a fetus by week 10. Miscarriage risk is highest in this trimester and falls sharply after week 12.
• Second trimester (weeks 14–27): Often called the "honeymoon trimester." Morning sickness typically eases, energy returns, and most people feel the baby's first movements (quickening) around weeks 16–20. The anatomy scan, usually at around week 20, checks fetal development in detail.
• Third trimester (weeks 28–40): The baby gains the majority of its birth weight during these weeks. Viability outside the womb improves steadily through this period. Provider visits become more frequent from around week 36 onward.

A full-term pregnancy typically lasts about 40 weeks from the first day of your last period.

Important Pregnancy Milestones

Beyond trimester boundaries, several specific gestational weeks carry clinical significance. Actual timing may vary slightly by provider and individual circumstance.

• Nuchal translucency (NT) scan — weeks 11–13: An ultrasound that measures the fluid at the back of the baby's neck. Combined with a blood test, it screens for chromosomal conditions such as Down syndrome. Timing is critical — this window closes after week 13.
• Anatomy scan — weeks 18–22: A detailed ultrasound that checks fetal organs and structures. Sex of the baby can usually be identified at this scan if desired.
• Glucose screening — weeks 24–28: A blood test to screen for gestational diabetes mellitus (GDM). Early detection allows dietary management that reduces risks for both mother and baby.
• Viability threshold — week 24: Fetuses born at this point have a chance of survival with intensive neonatal care. Survival rates increase significantly with each additional week.
• Early term — week 37: The baby is considered early term from 37 weeks. While outcomes are generally good, the final weeks of pregnancy are important for lung and brain maturation.
• Full term — week 39–40: The ideal delivery window. Most providers will not recommend induction before 39 weeks without a medical indication.

Full Term, Early Term, Late Term, and Post-Term Definitions

ACOG (American College of Obstetricians and Gynecologists) uses the following classifications for gestational age at delivery:

Delivery before 37 weeks is classified as preterm and carries higher risks that increase the earlier the birth occurs.

Frequently Asked Questions

• How accurate is this due date calculator?

  This calculator uses standard Naegele's rule and clinical conventions recommended by ACOG, and adjusts for your cycle length or ultrasound gestational age. However, only about 5% of babies are born on their exact due date. Delivery typically falls within a 2-week window before or after the EDD. Ultrasound dating by your provider — especially in the first trimester — is more precise than any self-reported calculation.

• Can I use this calculator if I have an irregular cycle?

  LMP-based estimates are less reliable when cycles are irregular or very short or long. Our calculator adjusts for cycle length within the 21–40 day range, but if your cycles vary significantly from month to month, ultrasound-based dating is more accurate because it measures the embryo directly rather than relying on assumed ovulation timing.

• What if my doctor's due date is different from this calculator's?

  Your provider may prioritise ultrasound-based dating or follow institutional dating guidelines. Clinical dates assigned by your OB-GYN or midwife — particularly those based on a first-trimester ultrasound — usually override self-calculated estimates. ACOG recommends revising the EDD if an early ultrasound differs from the LMP-based date by more than 7 days.

• Does every pregnancy last exactly 40 weeks?

  No. Full term is a range, roughly 37 to 42 weeks from LMP. The 40-week due date represents the 50th percentile estimated delivery date, not a fixed endpoint. Healthy babies are born across the full 37–42 week window. Normal biological variation in pregnancy length is considerable.

• How early can I find out my due date?

  An LMP-based estimated due date can be calculated as soon as you know the first day of your last period. If you're using the conception method, you need to know or estimate the date of ovulation. Ultrasound-based dating is most accurate in the first trimester — ideally before 14 weeks — and gives the most reliable EDD of all three methods.

• What counts as "full term"?

  According to ACOG, full term is 39 weeks 0 days through 40 weeks 6 days. The broader term "term pregnancy" covers 37 to 42 weeks. Delivery before 37 weeks is preterm, 37–38 weeks is early term, 41 weeks is late term, and 42 weeks or beyond is post-term. Each classification carries different management recommendations.

• Can I still be pregnant if my due date has passed?

  Yes — going a few days or even a week past your due date is very common. Only about 5% of babies arrive on their exact due date. Beyond 41 weeks, your provider will typically monitor more closely and discuss induction options. Most providers recommend induction by 41–42 weeks to avoid the small but real risks associated with post-dates pregnancy, such as placental insufficiency.

• What if I don't know my last period date?

  If you don't remember your LMP, try the conception date method (if you were tracking ovulation) or the ultrasound method. If you haven't had an ultrasound yet, your first prenatal appointment will include one — your provider will assign a gestational age and EDD from that scan.

• What does "gestational age" mean?

  Gestational age is counted from the first day of your last menstrual period, not from conception. A pregnancy at "8 weeks gestational age" is approximately 6 weeks post-conception. This LMP-based convention is the worldwide standard used by OB-GYNs, even though it counts roughly 2 weeks before fertilisation actually occurred.

• Can my due date change after it has been set?

  Yes. Your provider may revise your EDD based on early ultrasound measurements, especially if the scan-based date differs from the LMP-based date by more than 7 days (first trimester) or 10–14 days (second trimester). ACOG recommends using ultrasound dating when such discrepancies exist.