Pregnancy Due Date Calculator
Estimate your due date and pregnancy timeline using LMP method.
About the Pregnancy Due Date Calculator
This pregnancy due date calculator estimates your baby's expected delivery date using Naegele's Rule, the standard method used by obstetricians worldwide since the 19th century. Enter the first day of your last menstrual period (LMP) and your average menstrual cycle length, and the tool returns your estimated due date, conception date, current gestational age in weeks, trimester, and days remaining until delivery.
Naegele's Rule, formulated by German obstetrician Franz Naegele in the 1800s, calculates the due date as LMP + 280 days (40 weeks). The 280-day figure assumes a 28-day menstrual cycle with ovulation occurring on day 14; for shorter or longer cycles, the calculation is adjusted by adding or subtracting the cycle-length difference from 28 days.
Only about 4-5% of babies are born on their actual due date — most arrive within two weeks before or after. The due date is a target, not a deadline. Babies born between 37 and 42 weeks are considered 'full term'; before 37 weeks is 'preterm' and may need special care; after 42 weeks is 'post-term' and typically requires induction.
How It Works
Naegele's Rule:
Due Date = LMP + 280 days + (cycle_length - 28) daysWhere LMP is the first day of the last menstrual period. The 280-day figure (40 weeks) is the average human gestation from LMP. The cycle-length adjustment accounts for the fact that ovulation occurs 14 days before the next period — a 35-day cycle ovulates on day 21 (not day 14), adding 7 days to the gestation from LMP.
Conception date (approximate): LMP + 14 days, assuming a 28-day cycle. For longer or shorter cycles, conception is approximately LMP + (cycle_length - 14) days.
Trimester boundaries:
- First trimester: weeks 0-12 (LMP to 12 weeks)
- Second trimester: weeks 13-26
- Third trimester: weeks 27-40 (or until delivery)
Gestational age is calculated as (today - LMP) / 7, rounded down. Pregnancy is counted in weeks from LMP, not from conception — so at conception you are '2 weeks pregnant' by obstetric convention.
Worked Examples
Suppose your LMP was January 1, 2024 and your cycle is 28 days.
- Due date: January 1, 2024 + 280 days = October 7, 2024
- Conception date (approx): January 1 + 14 = January 15, 2024
- If today is May 1, 2024: gestational age = (May 1 - Jan 1) / 7 = 121 / 7 = 17 weeks
- Trimester: Second (weeks 13-26)
- Days remaining until due date: 159 days
If your cycle is 35 days instead of 28:
- Adjustment: 35 - 28 = +7 days
- Due date: October 7 + 7 = October 14, 2024
- Conception date: January 1 + (35-14) = January 22, 2024
If your cycle is 21 days (shorter):
- Adjustment: 21 - 28 = -7 days
- Due date: October 7 - 7 = September 30, 2024
First-trimester ultrasound measurement (crown-rump length) is the most accurate dating method, with error of just ±5-7 days. LMP-based dating has an error of ±14 days.
When to Use This Tool
Use this pregnancy due date calculator when:
- Estimating your due date after a positive pregnancy test
- Tracking your gestational age week by week throughout pregnancy
- Scheduling prenatal appointments (typically 8, 12, 20, 28, 32, 36, 40 weeks)
- Planning maternity leave start dates and work handover
- Determining which trimester you are in and what to expect developmentally
- Setting up a pregnancy journal or baby registry timeline
- Comparing LMP-based dating to first-trimester ultrasound dating
For the most accurate due date, your obstetrician will use a first-trimester ultrasound (crown-rump length measurement) to confirm or adjust the LMP-based estimate.
Limitations & Disclaimer
This calculator uses Naegele's Rule (LMP + 280 days + cycle-length adjustment), which has an error of approximately ±14 days compared to actual delivery. It assumes regular menstrual cycles of 21-35 days and ovulation occurring 14 days before the next period. It is less accurate for irregular cycles, very short or very long cycles, women who conceived shortly after stopping hormonal contraception (cycles may still be regulating), or pregnancies conceived via IVF (where the conception date is known exactly). The calculator does not replace first-trimester ultrasound dating, which is more accurate (±5-7 days) and is the standard of care for pregnancy dating. Estimated conception date is approximate; actual ovulation varies. The trimester boundaries shown are conventional but may differ slightly from your provider's classification. This is an educational tool, not medical advice. Confirm your due date with your obstetrician or midwife. See our disclaimer for full terms.
Frequently Asked Questions
How accurate is Naele's Rule?
Naegele's Rule has an error of about ±14 days compared to actual delivery. Only about 4-5% of babies are born on their exact due date; about 50% deliver within a week of it, and 90% within two weeks. First-trimester ultrasound (crown-rump length) is more accurate (±5-7 days) and is the preferred method when available.
Why is pregnancy counted from LMP, not conception?
Obstetric convention counts from LMP because it is observable (you know when your period started) while conception is invisible (you don't feel ovulation). Conception occurs about 14 days after LMP in a 28-day cycle, so you are technically '2 weeks pregnant' at conception. Gestational age (from LMP) is always 2 weeks greater than fetal age (from conception).
What if my cycles are irregular?
Naegele's Rule is less accurate with irregular cycles. If your cycles vary by more than 7 days, the LMP-based estimate may be off by 2-3 weeks. A first-trimester ultrasound is strongly recommended for irregular cycles, as it dates the pregnancy from fetal size rather than cycle history.
What is the difference between gestational age and fetal age?
Gestational age is counted from LMP (the obstetric standard) and is always 2 weeks more than fetal age. Fetal age (or conceptual age) is counted from conception. At '12 weeks pregnant' (gestational), the fetus is 10 weeks old (fetal). Medical literature uses both conventions; clarify which one is being referenced.
When is a baby considered full term?
Full term is 39-40 weeks. Babies born 37-38 weeks are 'early term' and may have slightly higher risks. Babies born before 37 weeks are 'preterm' and may need NICU care. Babies born 41-42 weeks are 'late term'; after 42 weeks is 'post-term' and typically requires induction due to declining placental function.
What happens if I go past my due date?
Most obstetricians induce labor between 41 and 42 weeks. After 42 weeks, the risk of stillbirth rises significantly due to placental aging and reduced amniotic fluid. Your provider will monitor fetal heart rate and amniotic fluid levels closely after 40 weeks and may recommend induction or a biophysical profile.
Last updated: July 21, 2026 · Author: HT99 Tools Editorial Team · Reviewed by: HT99 Tools Editorial Team