Last updated: 2026-05-12

Estimating Your Due Date

An estimated due date (EDD) gives expectant parents and healthcare providers a shared reference point for monitoring fetal development, scheduling prenatal appointments, and planning for delivery. It is calculated from a simple rule devised in the nineteenth century and still used universally today.

Naegele's Rule

The standard method is Naegele's rule, named after German obstetrician Franz Karl Naegele (1778–1851):

$\text{EDD} = \text{LMP} + 280 \text{ days}$

where LMP is the first day of the last menstrual period. 280 days equals 40 weeks, or roughly 9 calendar months and 7 days. The rule assumes a regular 28-day cycle with ovulation occurring around day 14.

Adjusting for Cycle Length

The textbook 280-day figure is exact only for a 28-day cycle. In a longer cycle, ovulation — and therefore conception — happens later, so the pregnancy reaches term later relative to the LMP. The standard correction shifts the estimate by the difference between your cycle and 28 days:

$\text{EDD} = \text{LMP} + 280 \text{ days} + (\text{cycle length} - 28)$

A regular 32-day cycle, for example, pushes the due date four days later; a 24-day cycle pulls it four days earlier. Enter your average cycle length and the calculator applies this adjustment automatically in the LMP method.

If you know your conception date rather than your LMP, the calculator converts it:

$\text{LMP equivalent} = \text{conception date} - 14 \text{ days}$

$\text{EDD} = \text{LMP equivalent} + 280 \text{ days} = \text{conception date} + 266 \text{ days}$

A known conception date already fixes ovulation timing, so no cycle-length adjustment is applied in conception mode. Both routes produce the same result for a 28-day cycle.

Why Gestational Age Is Counted From the LMP

Conception — the moment a sperm fertilises an egg — is almost never observed directly. The LMP, by contrast, is a reliably recalled and clinically measurable date. By convention, gestational age is therefore counted from the LMP, which means a newborn at 40 weeks of gestational age has an embryonic age of only about 38 weeks. This two-week offset is built into all standard due-date calculations.

$\text{Gestational age (days)} = \text{reference date} - \text{LMP}$

$\text{Gestational weeks} = \left\lfloor \frac{\text{gestational days}}{7} \right\rfloor$

Here ⌊·⌋ is the floor function — truncates to the completed number of whole weeks, discarding the remainder. The remaining days after dividing by 7 give the "plus days" in the standard clinical notation "Xw Yd".

The Three Trimesters

Pregnancy is conventionally divided into three trimesters:

The boundaries are approximations used for clinical communication, not sharp biological transitions.

Accuracy and Limitations

Naegele's rule is a population average, not a personalised prediction:

• Only ~5 % of babies are born on their exact EDD. Most births occur within two weeks on either side.
• Cycle length is accounted for, but irregular cycles are not. The calculator adjusts for a consistent cycle length other than 28 days, but if your cycles vary substantially from month to month, even the adjusted LMP estimate is uncertain.
• Ultrasound refines the estimate. A first-trimester ultrasound measuring crown-rump length is often more accurate than the LMP method, particularly when cycle length is uncertain or irregular. Your healthcare provider may update your EDD after an early scan.
• IVF and known conception dates allow the conception-date mode here to be more precise than the LMP mode, because ovulation timing is exactly known.

This calculator provides an informational estimate. For personalised guidance, consult your obstetrician, midwife, or general practitioner.