Last updated: April 2026 · Reviewed by Sarah Chen, RD

Pregnancy Due Date Calculator — Naegele's Rule, IVF & Ultrasound

Calculate your estimated due date using four standard obstetric methods: LMP (Naegele's rule), conception date, IVF embryo transfer date, or ultrasound with known gestational age. The calculator also shows trimester boundaries, key clinical milestones, current gestational age, and days remaining. Only about 4% of babies are born exactly on their due date — the result is an estimate, not a guarantee. Always confirm with your healthcare provider.

Method:          Naegele's Rule (LMP)
LMP date:        Sunday, May 3, 2026
Cycle length:    28 days
Cycle adjustment:+0 days (cycle − 28)
Due date:        LMP + 280 days + (0) = Sunday, February 7, 2027
Est. conception: LMP + 14 days + (0) = Sunday, May 17, 2026

This tool provides estimates only. Consult your healthcare provider for medical guidance.

Naegele's Rule — The LMP Method

The most widely used method for estimating a due date is Naegele's rule, named after German obstetrician Franz Karl Naegele who first described it around 1812. The formula is straightforward:

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

Naegele's rule is the default because the LMP is typically the most reliable date a pregnant person can accurately remember. It is also the date that obstetricians have used for over two centuries, making it the common language for discussing gestational age.

The rule assumes a standard 28-day menstrual cycle with ovulation occurring on day 14. If your cycle is consistently longer or shorter than 28 days, the expected ovulation date shifts accordingly, and the due date must be adjusted. Our calculator applies this correction automatically: for a 35-day cycle, the due date moves 7 days later; for a 21-day cycle, it moves 7 days earlier.

The 266-Day Conception Method

If you know the exact date of conception — for example, because you tracked ovulation with LH strips or temperature charting — a more direct calculation is available:

Due date = Conception date + 266 days (38 weeks)

This is equivalent to Naegele's rule but eliminates the two-week assumption between LMP and ovulation. In practice, however, most couples cannot pinpoint the exact day of conception, which is why the LMP method remains more common. The conception method is most useful when LMP is unknown or unreliable.

Note that 266 days (38 weeks from conception) equals 280 days (40 weeks from LMP) because the LMP method adds roughly 14 days to account for the pre-ovulatory phase of the cycle. Both formulas yield the same result when the cycle is exactly 28 days.

IVF Transfer Dates

In vitro fertilisation (IVF) pregnancies have a significant advantage when it comes to due date accuracy: the embryo's developmental age is known to the day at the time of transfer.

• Day 3 embryo transfer: embryo is 3 days past fertilization → due date = transfer date + 263 days
• Day 5 blastocyst transfer: embryo is 5 days past fertilization → due date = transfer date + 261 days
• Day 6 blastocyst transfer: embryo is 6 days past fertilization → due date = transfer date + 260 days

All three figures are derived from the same underlying principle: the embryo needs 266 days from fertilization (38 weeks) to reach full term. Because the embryo's age at transfer is already known, you simply subtract that from 266 to get the remaining days needed. IVF due dates are typically confirmed by a viability ultrasound at around 6–8 weeks after transfer.

Worked Example: Due Date from April 1, 2026 LMP, 28-Day Cycle

LMP date:         April 1, 2026
Cycle length:     28 days (standard — no adjustment)
Naegele's rule:   April 1, 2026 + 280 days

Step 1 — Count 280 days from April 1, 2026:
  April has 30 days → 29 remaining in April from April 1
  + May 31 = 60 days
  + June 30 = 90 days
  + July 31 = 121 days
  + August 31 = 152 days
  + September 30 = 182 days
  + October 31 = 213 days
  + November 30 = 243 days
  + December 31 = 274 days
  + January 6 = 280 days

Due date:         Wednesday, January 6, 2027

Back-calculate conception:
  LMP + 14 days = April 1 + 14 = April 15, 2026 (est. ovulation/conception)

Trimester boundaries:
  First trimester ends:  Week 13 → June 30, 2026
  Second trimester ends: Week 27 → September 28, 2026
  Third trimester:       Week 28 to due date

Trimester Markers & Pregnancy Milestones

Pregnancy is divided into three trimesters. While the exact week boundaries vary slightly by source, the most widely used divisions are:

Clinical Milestone Timeline

• Week 8–10: First (viability) ultrasound. Confirms heartbeat and gestational age.
• Week 10–12: NIPT (non-invasive prenatal testing) — blood test for chromosomal abnormalities including Down syndrome, trisomy 18, and trisomy 13. Can also reveal fetal sex.
• Week 11–14: First-trimester screening (nuchal translucency ultrasound + blood test).
• Week 20: Anatomy scan — detailed structural survey of the fetus. Checks all major organs, limbs, and placenta position.
• Week 24: Viability threshold. Neonates born at 24 weeks have meaningful survival rates with intensive neonatal support.
• Week 24–28: Glucose challenge test (GCT) or glucose tolerance test (GTT) — screens for gestational diabetes.
• Week 28: Rh-negative mothers typically receive anti-D immunoglobulin injection.
• Week 35–37: Group B Streptococcus (GBS) swab. If positive, IV antibiotics are given during labour to protect the newborn.
• Week 37: Full term begins. Birth at 37 weeks is no longer considered premature.
• Week 39–40: "Optimal" term — lowest risk for neonatal respiratory problems.
• Week 41–42: Post-term. Induction of labour is typically discussed from 41 weeks due to increased risk of placental insufficiency.

Why Only 4% of Babies Are Born on Their Due Date

The 280-day due date is an average, not a prediction for any individual pregnancy. The actual distribution of birth timing follows a bell curve centered around 40–41 weeks of gestation, with significant natural variation:

• Approximately 4–5% of babies are born exactly on the due date.
• About 50% of births occur within one week before or after the due date.
• About 80% occur within two weeks of the due date.
• Full term is defined as weeks 37–42 (259–294 days from LMP).
• Post-term pregnancies (past 42 weeks) carry increased risk and typically warrant medical review.

The bell curve peaks at around 40–41 weeks for first-time mothers (primigravidas), and slightly earlier for subsequent pregnancies. Factors such as genetic predisposition, ethnicity, and maternal age influence timing. The due date is therefore best understood as the midpoint of a normal range, not a deadline.

When to Be Concerned About Accuracy

LMP-based calculations are least reliable when any of these apply:

• Irregular cycles (shorter than 21 days or longer than 35 days): ovulation timing is unpredictable, making the 14-day ovulation assumption invalid.
• Unknown LMP: some people do not track their periods, or the last period was unusually light or irregular.
• Mid-cycle bleeding mistaken for a period: implantation bleeding or breakthrough bleeding can be confused with a period, leading to an incorrect LMP date.
• Recent hormonal contraception: cycles often take several months to regularise after stopping the pill or hormonal coil, making LMP less representative of the true cycle pattern.

In these situations, a dating ultrasound — ideally between 6 and 10 weeks — provides the most accurate gestational age. At this stage, fetal crown-rump length (CRL) is measured and varies minimally between fetuses. After 12 weeks, individual variation in fetal size increases, reducing ultrasound accuracy for dating.

Important Medical Note

This calculator provides an estimated due date based on standard obstetric formulas widely used in clinical practice. It is provided for informational purposes only and is not a substitute for medical advice. Individual pregnancies vary, and your healthcare provider will confirm and potentially adjust your due date using clinical examination and ultrasound. If you have concerns about your pregnancy, contact your midwife, obstetrician, or GP promptly.

Frequently Asked Questions