How due date calculators estimate your delivery date, the difference between conception date and last period methods, and what to expect from pregnancy weight gain guidelines.
The standard method for estimating a due date is called Naegele's Rule, named after the German obstetrician who developed it. It calculates an estimated due date as 280 days (40 weeks) from the first day of your last menstrual period, assuming a typical 28-day menstrual cycle.
This is the method most due date calculators and clinicians use as a starting estimate, since the first day of the last period is usually easier for most people to identify precisely than the actual date of conception.
Get an estimated due date based on your last menstrual period.
Open Due Date Calculator →If you know your specific conception date — for example, through IVF or precise ovulation tracking — a more direct calculation can be used: 266 days (38 weeks) from the date of conception. This avoids the assumption of a perfectly typical 28-day cycle that Naegele's Rule relies on, which can make it more accurate for people with longer or shorter natural cycles.
| Method | Calculation | Best for |
|---|---|---|
| Last menstrual period (Naegele's Rule) | 280 days from LMP | Most pregnancies — standard clinical method |
| Conception date | 266 days from conception | Known conception date (IVF, precise ovulation tracking) |
A more precise estimate if you know your specific conception date.
Open Conception Calculator →It's worth setting realistic expectations: a due date calculator produces an estimate, not a guarantee. Only a small minority of babies are actually born on their precisely calculated due date. Most healthy pregnancies deliver somewhere within roughly a 1-2 week window either side of the estimate — this is entirely normal and not a cause for concern on its own.
Recommended pregnancy weight gain depends on your pre-pregnancy BMI category — there isn't one single target that applies to everyone:
| Pre-pregnancy BMI | Recommended Total Gain |
|---|---|
| Underweight (under 18.5) | Generally higher — around 12.5-18kg |
| Healthy weight (18.5-24.9) | Generally around 11.5-16kg |
| Overweight (25-29.9) | Generally lower — around 7-11.5kg |
| Obese (30+) | Generally lowest — around 5-9kg |
See how your weight gain compares to general guidelines for your pre-pregnancy BMI.
Open Pregnancy Weight Gain Calculator →