Due Date Basics
- •Pregnancy is dated as 280 days (40 weeks) from the first day of your last period — not from conception.
- •That means you are counted as 2 weeks pregnant at the moment of conception.
- •A cycle longer than 28 days shifts your due date later by the same number of days.
- •Only about 4% of babies are born on their exact due date.
The Real Problem This Solves
The confusing part of due dates is that the count starts before you were pregnant. Pregnancy is dated from the first day of your last menstrual period, roughly two weeks before conception, which is why you are already "4 weeks pregnant" when a test first turns positive.
The second problem is cycle length. The standard 280-day rule assumes a textbook 28-day cycle with ovulation on day 14. If your cycles run 32 days, you ovulated later, and a calculator that ignores that will give you a due date several days too early. This one adjusts for it.
How the Due Date Is Calculated
The method is Naegele's rule: due date = first day of last period + 280 days, then shifted by (your cycle length − 28).
Example: With a last period starting 1 March and a standard 28-day cycle, the due date is 6 December. On a 32-day cycle the same start date gives 10 December — four days later, because ovulation happened four days later. On a 24-day cycle it moves earlier, to 2 December.
You may also see the shortcut "subtract three months, add seven days." That is a rough version of the same rule and can land a day or two off the exact 280-day count, because calendar months are unequal in length.
| Milestone | Weeks | Days from LMP | Why It Matters |
|---|---|---|---|
| Estimated conception | 2w 0d | 14 | Roughly ovulation — pregnancy is already dated as 2 weeks |
| End of first trimester | 13w 6d | 97 | Miscarriage risk drops substantially |
| Viability milestone | 24w 0d | 168 | Survival outside the womb becomes possible with intensive care |
| End of second trimester | 27w 6d | 195 | Third trimester begins |
| Early term begins | 37w 0d | 259 | No longer considered preterm |
| Full term begins | 39w 0d | 273 | The ideal delivery window opens |
| Estimated due date | 40w 0d | 280 | The headline date |
| Post-term | 42w 0d | 294 | Most providers induce before this point |
Frequently Asked Questions
Why does pregnancy start counting two weeks before I conceived?
Because the last menstrual period is the one date almost everyone can identify reliably, while the exact day of conception usually cannot be pinned down. Dating from the LMP gives a consistent standard, with the side effect that you are counted as about 2 weeks pregnant at conception and about 4 weeks when a home test first turns positive.
My cycles are irregular. Is this calculator still accurate?
Less so. The whole method assumes ovulation happened at a predictable point in your cycle, which is exactly what an irregular cycle breaks. If your cycles vary widely, an early ultrasound dating scan — most accurate in the first trimester — will give a far better date than any calculation from your LMP.
My doctor gave me a different due date. Which one is right?
Your provider's. A dating ultrasound measures the baby directly, and when it differs from the LMP-based estimate by more than about a week, clinical guidelines say the scan date replaces the calculated one. Treat this calculator as an estimate to orient yourself, not as a second opinion.
What is the difference between "full term" and the due date?
Full term is a window, not a day: 39 weeks 0 days through 40 weeks 6 days is considered full term, with 37-38 weeks classed as early term and 41 weeks onward as late term. The 40-week due date is simply the midpoint marker, which is why so few babies actually arrive on it.