Due Date Calculator
Calculate your estimated due date from your last period, ultrasound, conception date, or IVF transfer. Track pregnancy milestones and trimester progress.
Your due date is calculated by adding 280 days (40 weeks) to the first day of your last menstrual period. This assumes a 28-day cycle.
Enter your information below for a personalized due date calculation.
Calculate Your Due Date
Average cycle is 28 days. Range: 22-44 days.
How Due Dates Are Actually Calculated
The standard method for calculating a due date is Naegele's rule, published in 1812: take the first day of your last menstrual period, add one year, subtract three months, and add seven days. This gives you a date 280 days (40 weeks) from your LMP.
Naegele's rule assumes a 28-day cycle with ovulation on day 14. If your cycle is shorter or longer, ovulation happens at a different time, and your due date shifts accordingly. A 35-day cycle means ovulation around day 21 — adding a full week to the standard calculation.
This is why cycle length matters. This calculator adjusts for non-standard cycles, and also accepts ultrasound dating, known conception dates, and IVF transfer dates — each with different accuracy.
Accuracy of Different Dating Methods
| Method | Accuracy | When to Use |
|---|---|---|
| First-trimester ultrasound | ± 5–7 days | Most accurate method — gold standard before 13 weeks |
| IVF / known conception | ± 1–3 days | Most precise, since fertilisation date is known exactly |
| LMP (regular 28-day cycle) | ± 2 weeks | Good estimate if you know your LMP and have regular cycles |
| LMP (irregular cycles) | ± 3–4 weeks | Least reliable — confirm with early ultrasound |
| Second-trimester ultrasound | ± 1–2 weeks | Less accurate than first trimester but still useful |
What this means for you: If your LMP-based due date and your first ultrasound due date differ by more than 7 days, most clinicians will go with the ultrasound date. The earlier the ultrasound, the more accurate it is — embryos grow at very predictable rates in the first trimester.
When Do Babies Actually Arrive?
Only about 5% of babies are born on their exact due date. Here's the actual distribution:
| Gestational Age | Classification | Percentage of Births |
|---|---|---|
| Before 37 weeks | Preterm | ~10% |
| 37–38 weeks | Early term | ~25% |
| 39–40 weeks | Full term | ~50% |
| 41 weeks | Late term | ~10% |
| 42+ weeks | Post-term | ~5% |
First-time mothers tend to deliver slightly later (average 40 weeks + 5 days). Second and subsequent pregnancies tend to be a few days earlier. These are averages — individual variation is wide.
Key Milestones Before Your Due Date
| Week | Milestone | Why It Matters |
|---|---|---|
| 12 | Dating scan | Most accurate due date estimate — can shift your date by up to 2 weeks |
| 20 | Anomaly scan | Detailed check of baby's development; often when parents learn the sex |
| 28 | Third trimester begins | Appointments increase to fortnightly; start thinking about birth plan |
| 34 | Maternity leave earliest start | You can start mat leave from week 29, but most wait until 34-36 |
| 36 | Hospital bag packed | Baby could arrive any time from now — don't leave it later |
| 37 | Full term | Baby is considered full term; lungs are mature |
| 41 | Induction offered | NHS typically offers induction at 41-42 weeks |
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How to use this tool
Select your calculation method (Last Period, Ultrasound, Conception, or IVF)
Enter the required date and any additional information
Click 'Calculate Due Date' to see results
Common uses
- Calculating your estimated due date
- Adjusting for irregular cycle lengths
- Comparing LMP and ultrasound dating
- Planning maternity leave timing
- Tracking pregnancy milestones
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