Due Date Calculator · Practical guide
Due Date Calculator for Irregular Cycles
A 35-day cycle pushes your due date a week later — here is how cycle length shifts the estimate.
At a glance
Why Cycle Length Moves Your Due Date
Naegele's rule assumes you ovulate on day 14 of a 28-day cycle. Longer cycles usually mean later ovulation and later conception, so the due date shifts later; shorter cycles shift it earlier. The adjusted formula: Due date = LMP + 280 days + (cycle length − 28).
Interactive tool
Due Date Calculator
Enter your own values, then compare the result with the examples below.
Trimester Dates
| Trimester | Start Date | End Date |
|---|---|---|
| First Trimester | -- | -- |
| Second Trimester | -- | -- |
| Third Trimester | -- | -- |
Worked Example
With a last period starting January 10, 2026: a standard 28-day cycle gives October 17, 2026. A 35-day cycle adds 7 days for October 24, 2026; a 24-day cycle subtracts 4 days for October 13, 2026. Same period, three different due dates — the cycle-length field in the calculator above handles the adjustment for you.
Adjustment Reference (LMP January 10, 2026)
| Cycle Length | Adjustment | Estimated Due Date |
|---|---|---|
| 21 days | −7 days | October 10, 2026 |
| 24 days | −4 days | October 13, 2026 |
| 28 days | 0 | October 17, 2026 |
| 35 days | +7 days | October 24, 2026 |
| 40 days | +12 days | October 29, 2026 |
What If Your Cycles Are Truly Irregular?
If your cycle varies by more than a few days month to month, no formula can pin down ovulation from the calendar alone. Use your best average cycle length here for a rough date, then rely on a first-trimester ultrasound — accurate to about 5-7 days — for the dating your provider will actually use. If the ultrasound and LMP estimates disagree by more than about a week, providers generally adopt the ultrasound date.
Frequently Asked Questions
How much does a 35-day cycle change my due date?
Seven days later than the standard estimate, because 35 − 28 = 7. Each day your average cycle differs from 28 shifts the due date by one day in the same direction.
What if I do not know my average cycle length?
Leave the field at 28 and treat the result as approximate. You can also check period-tracking apps for your historical average, or wait for a dating ultrasound in the first trimester.
Does irregular ovulation make the due date wrong?
It makes the LMP-based estimate less reliable, since the formula guesses ovulation timing from cycle length. When cycles are highly irregular, an early ultrasound is the dependable way to date the pregnancy.
Can breastfeeding or coming off birth control affect the estimate?
Yes. Cycles right after stopping hormonal birth control or while breastfeeding are often irregular or ovulate late, so LMP dating is shakier. Tell your provider — they may weight the ultrasound more heavily.