Medical Information
The information on this page is for educational and informational purposes only. It is not medical advice and should not be used to diagnose or treat any medical condition. Always consult your healthcare provider (doctor, midwife, or nurse) before making any decisions about your pregnancy or your baby's health.
toolsHow Is a Due Date Calculated and How Accurate Is It?
How Naegele's rule and ultrasound dating work, why only 4% of babies arrive on their due date, what a dating scan changes, and how IVF due dates differ.
An estimated due date is one of the first pieces of information you receive in pregnancy, and it quickly becomes a focal point — circled on calendars, counted down on phones, shared with family. Yet the number carries far more uncertainty than it appears to. Understanding how that date is calculated, and what it actually means in practice, helps to set realistic expectations and reduces the anxiety that builds when things do not go exactly to plan.
Fact-Checked & Transparent: This article is fact-checked against current CDC, WHO, ACOG, and NHS guidelines. All health information is based on authoritative medical sources. Last verified: June 2026.
⚠️ MEDICAL DISCLAIMER
This article is educational information only. It is NOT a substitute for professional medical advice, diagnosis, or treatment.
Always consult your pediatrician or healthcare provider before making any medical decisions for your baby or child. Every baby is unique, and professional medical guidance is essential.
In case of emergency, call 911 or your local emergency number.
The Two Methods of Calculating a Due Date
Naegele's Rule: Calculating from the Last Menstrual Period
The oldest and most widely used method of estimating a due date is Naegele's rule, developed by German obstetrician Franz Karl Naegele in the early nineteenth century. The calculation is straightforward: take the first day of the last menstrual period (LMP), add 280 days — which is 40 weeks — and the result is the estimated due date (EDD).
In practice, this is sometimes simplified as: add one year, subtract three months, add seven days.
The reason 280 days is used rather than counting from conception is that conception typically occurs around two weeks after the first day of the last period in a standard 28-day cycle. So 40 weeks of pregnancy actually includes approximately two weeks before conception happened.
Naegele's rule has been used in clinical practice for nearly two centuries because it is simple, requires no equipment, and produces a reasonable estimate for many women. The NHS uses it as a starting point for all pregnancies.
Ultrasound Dating: The Gold Standard
Ultrasound dating has become the most accurate method of calculating a due date and is now considered the gold standard in clinical practice. Unlike Naegele's rule, ultrasound dating does not depend on the woman knowing when her last period was or having a 28-day cycle. Instead, it measures the baby directly.
The measurement used in early pregnancy is crown-rump length (CRL) — the distance from the top of the baby's head to the bottom of their spine. Between 11 and 14 weeks of pregnancy, crown-rump length is highly predictive of gestational age. This is because all babies at this stage are growing at a very similar rate, regardless of genetic factors that will later influence size. The CRL is compared against standardised growth charts to calculate gestational age and, from that, the EDD.
The NHS dating scan — offered to all pregnant women between 11 and 14 weeks — uses this measurement. If the ultrasound-derived date differs significantly from the LMP-based estimate, the scan date takes precedence. A difference of more than 7 days before 14 weeks typically results in the due date being revised.
Why LMP-Based Calculation Can Be Wrong
Naegele's rule contains a built-in assumption: that every woman has a 28-day cycle and ovulates on day 14. For many women, neither is true.
Irregular cycles mean that ovulation can occur considerably earlier or later than day 14. A woman with a 35-day cycle typically ovulates around day 21, which means Naegele's rule would underestimate her gestational age and give a due date that is too early.
Long cycles have the opposite problem. If cycles are consistently 35 or 40 days, calculating 280 days from the LMP over-estimates how far along the pregnancy is.
Not knowing the LMP is more common than it might seem. Around 10–40% of women cannot reliably identify the first day of their last period, particularly those with irregular cycles or those who were not tracking their cycle at the time.
Conception via assisted reproduction produces varying scenarios depending on the method used (see the IVF section below).
These sources of error are exactly why ultrasound dating was adopted as the standard. A scan cannot be misled by cycle length or ovulation timing — it simply measures the baby.
The Dating Scan: What It Does and Why It Matters
The NHS dating scan, typically scheduled between 11 and 14 weeks, serves multiple purposes. Dating the pregnancy and confirming the EDD is one of them, but the scan also:
- Confirms that the pregnancy is in the uterus (ruling out ectopic pregnancy)
- Checks for a heartbeat and fetal viability
- Identifies twin or multiple pregnancies
- Measures the nuchal translucency (NT) fold as part of the combined screening test for chromosomal conditions such as Down's syndrome (when combined with a blood test)
The crown-rump length measurement taken at this scan is the most accurate single predictor of due date available. Studies have shown that ultrasound dating at 11–14 weeks can correct a LMP-based estimate by as much as two weeks in some cases, which has significant implications for decisions about induction later in pregnancy.
After 20 weeks, ultrasound measurements become less reliable for dating purposes because fetal size begins to reflect individual genetic variation rather than gestational age alone. This is why earlier scans are always preferred for establishing the EDD.
How Accurate Is a Due Date?
The short answer is: not as accurate as it feels.
Only approximately 4% of babies are born on their estimated due date. Research consistently shows that around 80% of pregnancies end within two weeks either side of the EDD — meaning the realistic window for "on time" delivery spans a full month.
The term "due date" implies a precision that the biology does not support. A more accurate framing would be "due month" or, as some researchers have proposed, a "due range" of 37 to 42 weeks.
Even with an accurate scan-derived EDD, individual variation in the length of pregnancy is significant. A study published in the journal Human Reproduction found that the length of pregnancy varied by as much as 37 days among women with confirmed ovulation dates — suggesting that biological differences in how long pregnancies last are substantial, independent of measurement error.
When GPs Give Different Dates
It is not unusual for a pregnant woman to receive slightly different estimates from her GP and from her dating scan. This happens because GPs often calculate the EDD from the LMP before the scan has taken place. Once the 11–14 week scan occurs and crown-rump length is measured, the scan-derived date replaces the LMP estimate as the official EDD on the maternity record.
If the scan date and LMP date differ by more than a week, this is not a cause for alarm. It most often reflects the normal variation in cycle length and ovulation timing discussed above. The scan date is used going forward for all subsequent clinical decisions.
Due Dates for IVF Pregnancies
IVF pregnancies are calculated differently because the date of egg retrieval — and often the date of embryo transfer — is known precisely. This removes the uncertainty that comes with calculating from the LMP in naturally conceived pregnancies.
For IVF with a fresh embryo transfer, the EDD is typically calculated as 266 days (38 weeks) from the date of egg retrieval, or equivalently 263 days from the date of a three-day embryo transfer, or 261 days from a five-day (blastocyst) transfer.
Because the conception date is known, IVF due dates are generally considered more accurate than LMP-derived dates. An early ultrasound is still performed to confirm the pregnancy is developing normally, but it is less likely to significantly revise the EDD.
What "Full Term" Actually Means
The term "full term" does not mean 40 weeks exactly. According to ACOG (the American College of Obstetricians and Gynecologists), pregnancy is classified as follows:
- Early term: 37 weeks 0 days to 38 weeks 6 days
- Full term: 39 weeks 0 days to 40 weeks 6 days
- Late term: 41 weeks 0 days to 41 weeks 6 days
- Post term: 42 weeks 0 days and beyond
Babies born at 37 or 38 weeks are technically "term" but are not developmentally equivalent to those born at 39 or 40 weeks. Research has shown that babies born at 39–40 weeks have better outcomes on average than those born at 37–38 weeks, which is why elective deliveries before 39 weeks are generally avoided unless medically necessary.
Post-Dates Pregnancy and Induction
When a pregnancy continues beyond the due date, it enters what is sometimes called a post-dates or prolonged pregnancy. In the UK, most hospitals offer induction of labour between 41 and 42 weeks, with the NHS recommending a discussion about induction from 41 weeks.
The reason induction is offered rather than waiting indefinitely is that risks begin to increase as pregnancy extends significantly past the due date. After 42 weeks, there is an increased risk of stillbirth, though the absolute risk remains low. Placental function can also become less efficient over time.
The decision about whether to accept induction or wait is a personal one and should be made with full information about both the risks of waiting and the risks of the induction process itself. Your midwife will offer a membrane sweep from around 40 weeks to try to encourage natural labour before induction becomes necessary.
The Emotional Weight of a Due Date
A due date quickly becomes more than a medical estimate — it becomes an anticipated event, a countdown, a social fact shared with everyone who asks when the baby is due. This is understandable, but it can create real distress when the date passes without labour starting.
Being mentally prepared for the fact that the due date is an approximation — not a deadline — can reduce the anxiety of those final weeks. It can help to think of the EDD as the midpoint of a realistic birth window, not the endpoint of pregnancy.
If you reach your due date and have not gone into labour, this is completely normal. Speak with your midwife about your options, and try not to treat the date as a sign that something has gone wrong.
Frequently Asked Questions
Can my due date change after the dating scan?
Yes. If the crown-rump length measured at the 11–14 week dating scan differs from the LMP-based estimate by more than 7 days, the due date will be revised to match the scan. This is normal and does not indicate a problem. The scan-derived date is considered more accurate.
What if I don't know when my last period was?
This is more common than many people expect. If you cannot reliably date your last period, your healthcare provider will rely on ultrasound measurements to establish the gestational age and EDD. An early dating scan is particularly important in this situation.
Does it matter which method is used to calculate the due date?
Yes, because clinical decisions — including when to offer induction — are based on the EDD. If the LMP-derived date is used but is inaccurate, a woman may be offered induction earlier or later than is appropriate. This is why scan-based dating is now standard practice in the NHS.
Is the due date different for twin pregnancies?
The EDD is calculated the same way for twins, but twins are at higher risk of preterm birth and are typically delivered before 40 weeks. The NHS recommends that uncomplicated dichorionic (non-identical) twins are delivered by 37 weeks and monochorionic twins earlier. Your midwife will discuss the specific plan for your twin pregnancy.
What happens if my baby hasn't arrived by 42 weeks?
Induction of labour will typically be offered. The risks of continuing the pregnancy beyond 42 weeks — including an increased risk of stillbirth and meconium aspiration — mean that most NHS trusts recommend induction by this point. You will be monitored closely and given information to help you decide.
Why do only 4% of babies arrive on their due date?
Because the due date is a statistical midpoint based on population averages, not a prediction specific to your individual pregnancy. Natural variation in the length of pregnancy is substantial, and even with an accurate EDD, there is a wide normal range for when labour begins.
Key Takeaways
- Naegele's rule adds 280 days to the first day of the last menstrual period and is the traditional calculation method, but it assumes a 28-day cycle and day-14 ovulation
- Ultrasound dating using crown-rump length at 11–14 weeks is the gold standard — it is not affected by cycle length and can correct the LMP estimate by up to two weeks
- Only approximately 4% of babies are born on their estimated due date; around 80% are born within two weeks either side
- IVF pregnancies use the date of egg retrieval for dating, making the EDD more precise than in naturally conceived pregnancies
- "Full term" spans 39–41 weeks — babies born at 37–38 weeks are technically term but are not developmentally equivalent
- Most UK hospitals offer induction between 41 and 42 weeks; the NHS recommends a discussion from 41 weeks onwards
📋 SOURCES & FACT-CHECKING
This article is compiled and verified against these authoritative sources: - CDC (Centers for Disease Control & Prevention) - WHO (World Health Organization) - ACOG (American College of Obstetricians and Gynecologists) - NHS (National Health Service) - AAP (American Academy of Pediatrics)
Last verified: June 2026
Educational content only. Always consult your pediatrician for medical decisions.
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PregnancySprout Editorial Team
Our editorial team researches every article against primary medical sources — NHS, WHO, NICE, and RCOG guidelines. We are health writers and parents, not doctors; content is reviewed for accuracy but does not constitute medical advice.
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