Pregnancy Due Date Calculator

How This Calculator Works

A pregnancy due date is an estimate of when a baby will be born, calculated from a known starting point plus a standard gestation length. Choose whichever starting point you actually know — the first day of your last menstrual period, a known conception date, or an IVF embryo transfer date — and this calculator estimates your due date, plus how far along you are right now (in weeks and days) and which trimester that falls in.

First day of last period uses Naegele’s rule, the standard estimation method most healthcare providers start from: it assumes a typical 28-day cycle with ovulation around day 14. It’s a widely-used starting estimate, not a prediction of the actual birth date — an early ultrasound is more accurate, especially if your own cycle doesn’t run close to 28 days.

Conception date and IVF transfer date don’t carry that same cycle-length assumption, since both start from an already-known date rather than an estimated ovulation day — an IVF transfer date in particular is typically the most precise of the three, since a fertility clinic knows the embryo’s exact age at transfer.

The Formula

From the last menstrual period:

$$\text{Estimated Due Date} = \vA{\text{First Day of Last Period}} + 280 \text{ days (40 weeks)}$$ $$\text{Estimated Conception Date} = \vA{\text{First Day of Last Period}} + 14 \text{ days}$$

From a known conception date:

$$\text{Estimated Due Date} = \vB{\text{Conception Date}} + 266 \text{ days (38 weeks)}$$

From an IVF embryo transfer date — the offset depends on the embryo’s age at transfer, a widely-published fertility-clinic convention:

$$\text{Estimated Due Date} = \vC{\text{Transfer Date}} + 261 \text{ days (Day 5 transfer)}$$ $$\text{Estimated Due Date} = \vC{\text{Transfer Date}} + 263 \text{ days (Day 3 transfer)}$$

Gestational age (how far along the pregnancy is on any given day) is simply the number of days since the first day of the last period (or its equivalent, worked backward from whichever starting point was actually entered), expressed in weeks and days.

Worked Example

Last menstrual period starting January 1, 2024:

1. Due date: $\vA{\text{January 1, 2024}} + 280 \text{ days} = \text{October 7, 2024}$.
2. Estimated conception date: $\vA{\text{January 1, 2024}} + 14 \text{ days} = \vB{\text{January 15, 2024}}$.
3. As of March 15, 2024 (74 days later), that’s 10 weeks, 4 days along — still in the first trimester (weeks 1-13).

The other two methods agree with this exact same example when their own dates line up with the same pregnancy: a conception date of January 15, 2024 gives $\vB{\text{January 15, 2024}} + 266 \text{ days} = \text{October 7, 2024}$ — the identical due date. A Day 5 IVF transfer on January 20, 2024 gives $\text{January 20, 2024} + 261 \text{ days} = \text{October 7, 2024}$ — again, the same date.