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 โ only about 4% of babies are born on their exact due date (Jukic et al., 2013) โ 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 โ and each method has a different level of precision.
How Is Pregnancy Calculated?
Pregnancy is counted from the first day of your last period, not from conception. That means you're already about 2 weeks "pregnant" when conception happens. A full-term pregnancy is counted as 40 weeks (280 days) from that date. Your due date is the point where 40 weeks is reached.
0โ13 weeks is the first trimester, 14โ27 the second and 28โ40 the third. Babies born from 37 to 42 weeks are considered full term.
The start of your last period is usually a date you know, while conception is a date you can't see.
Quick example: last period started 1 Jan 2026 โ due date 8 Oct 2026 (1 Jan + 280 days).
See how many weeks pregnant you are, the gestational age calculator, gestational age vs fetal age, and when was I conceived.
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 โ from ovulation tracking, a fertility monitor, or a single instance of intercourse โ 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. Use the conception date calculator if you already know that date.
Method 3: Ultrasound Dating
A first-trimester dating ultrasound measures the embryo or fetus (typically crown-rump length between 6โ13 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 5โ7 days (the exact threshold depends on gestational age), most clinicians will use the ultrasound date instead โ early ultrasound measurements are generally considered more accurate than LMP-based calculations alone. ACOG says measuring the embryo or fetus by ultrasound in the first trimester (up to 13 weeks 6 days) is the most accurate way to set or confirm gestational age. In England, the dating scan is offered at around 10 to 14 weeks (NHS). Use the ultrasound due date calculator if you have a scan measurement.
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 263 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. For a full fresh and frozen embryo transfer (FET) walkthrough, see how to calculate your FET due date. To run day 3 or day 5 dating with equivalent LMP and gestational age, use the IVF due date calculator.
Comparison of Due Date Methods
| Method | What You Need | Typical Precision | Best For |
| Last Menstrual Period | LMP date, average cycle length | Moderate โ 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 โ exact fertilization timing known | IVF pregnancies |
How Accurate Are Pregnancy Due Dates?
Due dates are estimates. Only about 4% of babies are born on their exact due date, and about 70% arrive within 10 days of it, even when the date comes from an ultrasound. A first-trimester dating scan gives the most accurate due date, so many clinics replace a period-based date with it.
Even with a known conception date, pregnancies vary. In a study where ovulation was measured directly, healthy full-term pregnancies varied in length by up to 37 days. The median time from ovulation to birth was 268 days (Jukic et al., 2013).
What makes a period-based date less accurate: irregular or long cycles, not remembering the date, or getting pregnant soon after stopping hormonal contraception or while breastfeeding.
When the date is set: ACOG says the due date should be set from your last period and/or the first accurate ultrasound, and later changes should be rare (ACOG Committee Opinion 700).
| Dating method | How accurate | Notes |
| Last period (Naegele's rule) | Good if cycles are regular | Less reliable with irregular cycles or an uncertain date |
| Conception or ovulation date | Good if the date is well tracked | Pregnancy length still varies naturally |
| First-trimester ultrasound (to 13w6d) | Most accurate for natural conception (ACOG) | Usually replaces the period date if they differ |
| IVF transfer date | Very precise starting point | Embryo age is known |
Why Your Due Date Might Change
It's common for a due date to shift slightly as a pregnancy progresses โ 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?
A due date is a good estimate, not a prediction. Only about 4% of babies arrive on the exact day, and about 70% are born within 10 days of it. A first-trimester dating scan gives the most accurate estimate.
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 โ say 35 days โ 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 6โ13 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 โ 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. See how to calculate your FET due date for the full IVF and FET walkthrough, or the IVF due date calculator for your exact transfer date.
What percentage of babies are born on their due date?
About 4%. Most babies are born in the weeks around the due date, and about 70% arrive within 10 days of it.
Is an ultrasound due date more accurate than one from my last period?
Usually, yes. ACOG considers a first-trimester ultrasound the most accurate way to date a pregnancy. If the two dates differ enough, your provider will normally use the ultrasound date.
Why is pregnancy 40 weeks and not 9 months?
Pregnancy is counted from the first day of your last period, about 2 weeks before conception. 40 weeks works out to about 9 months plus a week.
Sources
- Jukic et al., 2013, Human Reproduction
- ACOG Committee Opinion 700
- NHS, 12-week (dating) scan
- NHS, Due date calculator
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.
Last updated: October 17, 2026