Pregnancy Due Date

Turning a Known Starting Point Into an Estimated Due Date

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.

Key Factors to Consider

• Only about 5% of babies are actually born on their exact estimated due date. A due date is a statistical estimate of the most likely delivery window, not a firm prediction — full-term delivery is normally considered to span roughly 3 weeks before to 2 weeks after the estimated date, so a due date is better thought of as the center of a range than a single expected day.
• An early first-trimester ultrasound is generally considered the most accurate way to date a pregnancy, especially when cycle length is irregular. Naegele’s rule assumes a textbook 28-day cycle with ovulation on day 14 — someone with a longer or shorter typical cycle will see this estimate drift from their actual timeline, which is exactly the scenario an early ultrasound measurement corrects for.
• The three starting-point methods (last period, conception date, IVF transfer) converge on the same underlying pregnancy timeline when they’re describing the same real pregnancy. They use different day-offsets specifically because they’re measuring from different reference points in that same timeline (menstrual period start vs. actual conception vs. a known-age embryo) — not because they represent competing estimates of the same starting point.
• Trimester boundaries are commonly cited in weeks, but sources vary slightly on the exact cutoffs. This calculator uses the widely-cited weeks 1-13 (first), 14-27 (second), and 28+ (third) breakdown — a healthcare provider’s own chart may define boundaries a week differently, which is a normal, harmless variation rather than a sign of an inconsistency.

Common Mistakes

• Treating the due date as a firm deadline rather than the center of a range. Since only about 5% of babies arrive on their exact due date, planning around a single day (rather than the roughly 3-weeks-before-to-2-weeks-after full-term window) can lead to unnecessary surprise or worry when delivery happens a bit earlier or later.
• Using the last-period method without accounting for an irregular cycle. Naegele’s rule assumes ovulation around day 14 of a 28-day cycle — someone with a notably longer or shorter cycle should treat this estimate as a rougher starting point and lean more on an early ultrasound or a known conception date.
• Mixing up a Day 3 and Day 5 IVF transfer when entering the transfer date. The two use different day offsets specifically because the embryo’s age at transfer differs — entering the wrong transfer day shifts the estimated due date by two days in the wrong direction.

Useful to Know

• Trying to pin down your conception date more precisely from cycle tracking? Ovulation Calculator estimates your fertile window and likely ovulation day.
• Tracking how much weight to expect to gain over the pregnancy? Pregnancy Weight Gain Calculator estimates a healthy weight-gain range based on starting BMI.
• Curious how tall your baby might grow up to be? Child Height Predictor Calculator estimates adult height from parents’ heights.