A due date is a calculated estimate, not a promise — and understanding the math behind it explains why your doctor might quote a slightly different date than an app, and why that date can shift after your first ultrasound. Here's exactly how it works.
The most widely used due date calculation method is named after Franz Naegele, the German obstetrician who popularised it in the 19th century. It's a simple, elegant rule: add 280 days (40 weeks) to the first day of your last menstrual period (LMP).
This formula assumes a typical 28-day menstrual cycle with ovulation around day 14. The underlying logic is really "266 days from conception plus roughly 14 days for the time before ovulation" — which is why gestational age and fetal age differ by about two weeks, covered next.
This distinction confuses many people the first time they encounter it. A pregnancy is described as lasting "40 weeks" — but the baby doesn't actually develop for 40 weeks from conception.
| Measure | Counted From | Total Length |
|---|---|---|
| Gestational age | First day of last menstrual period (LMP) | ~40 weeks (standard convention) |
| Fetal (conceptional) age | Actual date of conception | ~38 weeks |
Doctors and pregnancy tracking conventionally use gestational age — it's what appears on scans, in prenatal appointments, and in every "you are X weeks pregnant" reference, even though conception itself happens roughly two weeks after the LMP date the count starts from.
| Method | Calculation | Best For |
|---|---|---|
| Last Menstrual Period (LMP) | LMP + 280 days | Standard method for regular cycles |
| Conception date | Conception date + 266 days | When conception date is known with confidence |
| IVF transfer date | Transfer date + 261 or 263 days (see below) | IVF pregnancies — generally the most precise |
LMP-based dating assumes a standard 28-day cycle — for people with longer, shorter, or irregular cycles, this method becomes noticeably less reliable, which is exactly when an early ultrasound measurement becomes valuable for confirming or adjusting the estimate.
IVF pregnancies have an advantage LMP-based dating doesn't: the embryo's exact developmental age is known precisely at the moment of transfer, removing the ovulation-timing guesswork entirely.
| Transfer Type | Formula |
|---|---|
| Day 5 transfer (blastocyst) | Transfer date + 261 days |
| Day 3 transfer | Transfer date + 263 days |
This accounts for the embryo's known age at transfer, which is why IVF-based due dates are generally considered more precise than LMP-based estimates.
| Trimester | Weeks |
|---|---|
| First | Weeks 1–12 |
| Second | Weeks 13–27 |
| Third | Weeks 28–birth (typically ~week 40) |
These boundaries are used consistently across most obstetric guidelines, though some sources define the exact week transitions slightly differently by a week either way.
Neha's last menstrual period started on 10 January 2026. Using Naegele's rule:
As of today, Neha would be tracked as a certain number of weeks pregnant (gestational age) counted from 10 January, placing her in whichever trimester that week falls into. Try ToolLoom's Pregnancy Due Date Calculator to get your exact due date, current gestational age, and trimester instantly from LMP, conception, or IVF transfer date.
It's worth setting realistic expectations upfront: only about 5% of babies are born on their exact calculated due date. Most arrive within a two-week window on either side — which is precisely why it's called an "estimated" due date, not a fixed one.
An early ultrasound, typically done in the first trimester, measures the fetus directly and is considered more accurate than LMP-based dating alone — especially for anyone with irregular cycles. If the ultrasound measurement differs meaningfully from the LMP estimate, doctors will often adjust the due date to match the ultrasound finding rather than the original LMP calculation.