Your estimated due date can be calculated four different ways — from your last period, conception date, an ultrasound scan, or an IVF transfer — and each method gives a different level of accuracy. Here's how each one works.
An estimated due date (EDD) is one of the first pieces of information most expectant parents want after a positive pregnancy test. It isn't a guaranteed delivery day — only about 4–5% of babies are born on their exact due date — but it's a useful clinical anchor for scheduling prenatal care, planning parental leave, and tracking fetal development against expected milestones.
What surprises a lot of people is that there isn't just one way to calculate a due date. Depending on what information you have available, your due date might be estimated from your last period, your conception date, a dating ultrasound, or an IVF embryo transfer date — and each method has a different level of precision.
Method 1: Last Menstrual Period (Naegele's Rule)
The most common starting point is the first day of your last menstrual period (LMP). This method, known as Naegele's Rule, adds 280 days (40 weeks) to the LMP, assuming a standard 28-day cycle with ovulation on day 14.
If your average cycle is longer or shorter than 28 days, the estimate can be adjusted: add the difference for longer cycles, subtract it for shorter ones. For example, a 32-day cycle pushes the estimate about 4 days later than the standard calculation.
Accuracy: Reasonably reliable for people with regular cycles, less reliable for irregular cycles, recent hormonal contraception use, or breastfeeding-related cycle changes.
Method 2: Conception Date
If you know roughly when conception occurred — from ovulation tracking, a fertility monitor, or a single instance of intercourse — the due date is estimated as 266 days (38 weeks) from that date. This skips the assumption about when ovulation happened relative to your last period, so it can be more precise than LMP-based dating for people who track ovulation closely.
Method 3: Ultrasound Dating
A first-trimester dating ultrasound measures the embryo or fetus (typically crown-rump length between 6–13 weeks) and compares it to standard growth charts to estimate gestational age. When the ultrasound estimate and the LMP-based estimate disagree by more than about 5–7 days (the exact threshold depends on gestational age), most clinicians will use the ultrasound date instead — early ultrasound measurements are generally considered more accurate than LMP-based calculations alone.
Method 4: IVF Transfer Date
For IVF pregnancies, the embryo's exact age at transfer is known, which removes the ovulation-timing guesswork entirely. A Day-5 blastocyst transfer adds 261 days to reach the due date; a Day-3 transfer adds 266 days. Because the moment of fertilization is documented, this is typically the most precise of the four methods — though individual clinics may use slightly different offsets.
Comparison of Due Date Methods
| Method | What You Need | Typical Precision | Best For |
| Last Menstrual Period | LMP date, average cycle length | Moderate — depends on cycle regularity | Anyone tracking their cycle, without other data |
| Conception Date | Known or estimated conception date | Good, if the date is well-tracked | People actively tracking ovulation |
| Ultrasound Dating | Scan date + measured gestational age | High, especially in the first trimester | Anyone who has had a dating scan |
| IVF Transfer Date | Transfer date, embryo age (Day 3 or 5) | Highest — exact fertilization timing known | IVF pregnancies |
Why Your Due Date Might Change
It's common for a due date to shift slightly as a pregnancy progresses — this doesn't mean anything went wrong. Common reasons include:
- A dating ultrasound revising an initial LMP-based estimate
- Longer or shorter cycles than the standard 28-day assumption
- Later-than-average ovulation
- Updated information (e.g., conception date confirmed after the fact)
Frequently Asked Questions
How accurate are due date calculators?
Due date calculators give a reliable estimate, but only about 4–5% of babies are born on their exact estimated due date. Most births occur between 38 and 42 weeks of gestation — a roughly two-week window around the EDD is the normal range, not the exception.
What is Naegele's Rule?
Naegele's Rule is the standard formula for estimating a due date from the last menstrual period: add 280 days (40 weeks) to the LMP, assuming a 28-day cycle with ovulation on day 14. It was developed by German obstetrician Franz Karl Naegele in the early 19th century and remains the most widely used starting point for due date estimation, though it's commonly adjusted for cycle length.
Does cycle length affect my due date?
Yes. The standard calculation assumes a 28-day cycle. If your average cycle is longer — say 35 days — ovulation likely happens later, which shifts your estimated conception date, and therefore your due date, later too. Tracking your cycle length for a few months beforehand makes this adjustment more reliable.
Why did my due date change after an ultrasound?
Early ultrasound measurements (particularly crown-rump length between 6–13 weeks) are generally more precise than LMP-based dating. If the two estimates disagree by more than about a week, most clinicians will use the ultrasound-based date going forward — this is a normal part of routine prenatal care, not a red flag.
How is an IVF due date different from a natural conception due date?
With IVF, both the transfer date and the embryo's exact age at transfer are known, which removes the uncertainty around ovulation timing entirely. That's why IVF-based due dates tend to be the most precise of any method — the moment of fertilization is documented rather than estimated.
Use the Pregnancy Due Date Calculator to try any of these four methods with your own dates, or browse the full set of women's health calculators for cycle, fertility, and pregnancy tracking tools.