Understanding Your Due Date: How It Is Calculated and Why It Can Change
A clear guide to how your due date is calculated, why it is an estimate rather than a deadline, and why it may be revised during pregnancy.
The due date is one of the first things established in pregnancy and one of the most misunderstood. From the moment it is given, it tends to take on a weight and precision it does not actually have — circled on calendars, counted down to, and treated as the date the baby will arrive.
The reality is that a due date is an estimate. It is a statistical midpoint based on the average length of pregnancy, calculated from a set of assumptions that do not apply equally to every woman or every pregnancy. Only about five percent of babies are born on their exact due date. The large majority are born within two weeks before or after it — and all of those births are, in clinical terms, at term and entirely normal.
Understanding how the due date is calculated, why it may be revised, and what it actually means for how your pregnancy is managed makes it a useful piece of information rather than a source of anxiety as the date approaches and passes.
How the due date is calculated
Naegele’s rule: calculation from the last menstrual period
The most traditional method of calculating a due date is Naegele’s rule, which works from the first day of your last menstrual period (LMP).
The formula: add seven days to the first day of your last menstrual period, then add nine months (or equivalently, subtract three months and add seven days). The result is the estimated due date.
Example: If your last period started on 1 January, your due date would be calculated as 8 October.
The logic behind this formula reflects the average length of a human pregnancy: approximately 280 days, or forty weeks, counted from the first day of the last menstrual period. Crucially, this includes approximately two weeks before ovulation and fertilisation actually occur — meaning that at the point of conception, you are already counted as being approximately two weeks pregnant in clinical terms.
This is why “weeks pregnant” in clinical pregnancy dating always refers to weeks from the last menstrual period, not weeks from conception. A woman who is described as six weeks pregnant has typically been pregnant for only about four weeks from the moment of fertilisation.
The assumptions Naegele’s rule makes — and why they do not always hold
Naegele’s rule assumes a regular twenty-eight-day menstrual cycle with ovulation on day fourteen. In practice, cycles vary considerably:
Longer cycles: A woman with a regular thirty-five-day cycle ovulates around day twenty-one, not day fourteen. Using LMP alone, her due date would be calculated as seven days earlier than it should be. She would appear to be further along than she is.
Shorter cycles: A woman with a twenty-one-day cycle ovulates earlier, and her LMP-based due date would underestimate her gestation.
Irregular cycles: Many Indian women have irregular cycles — including those with polycystic ovary syndrome (PCOS), which is extremely prevalent in South Asia. Irregular cycles make LMP-based dating unreliable.
Uncertain LMP: Many women cannot reliably recall the first day of their last period, particularly if cycles were irregular or if contraception was recently stopped.
Conception after fertility treatment: Women who conceived through IVF or other fertility treatments know their conception date precisely, and dating from LMP is either unnecessary or adjusted accordingly.
Ultrasound dating: the more accurate method
Ultrasound dating — measuring the baby directly to determine gestational age — has largely superseded LMP-based dating as the primary method when a first-trimester scan is available.
In early pregnancy, all embryos grow at a remarkably similar rate, regardless of the mother’s genetics or ethnicity. This means that measuring the embryo’s size in early pregnancy is a reliable and accurate way to determine how far along the pregnancy is — more accurate than LMP calculation for most women.
Crown-rump length (CRL): The primary measurement used in first-trimester dating. Measured on ultrasound from the top of the baby’s head (crown) to the bottom of the spine (rump), excluding the legs. CRL is most accurate between seven and thirteen weeks.
Dating accuracy by timing:
- At seven to nine weeks: accurate to plus or minus three to five days
- At ten to thirteen weeks: accurate to plus or minus five to seven days
- At fourteen to twenty weeks: accurate to plus or minus seven to ten days
- After twenty weeks: accuracy decreases, as fetal size becomes more variable between individuals
This is why the first-trimester scan is the most important dating scan in pregnancy. If you have a first-trimester ultrasound, the due date it establishes is more reliable than one based on LMP alone — and most providers use it to confirm or revise the LMP-based date.
Why your due date may change
After a first-trimester ultrasound
The most common reason a due date changes is that a first-trimester ultrasound produces a date that differs from the LMP-calculated one.
When the difference is small — five days or fewer — the LMP date is usually kept. When the difference is larger, the ultrasound date takes precedence, because it is the more accurate measurement.
For women with irregular cycles or uncertain LMP, the first ultrasound date often produces a due date that differs significantly from the LMP estimate. This is not an error — it is a correction toward accuracy.
After a dating scan if no first-trimester scan was done
If a woman presents for her first antenatal care in the second trimester without a first-trimester scan, ultrasound dating at that point provides a less precise but still useful estimate. The measurements used change:
- Second-trimester dating uses head circumference, abdominal circumference, and femur (thigh bone) length combined to estimate gestational age.
- Accuracy at this stage is approximately plus or minus ten to fourteen days — wider than first-trimester dating, reflecting the increasing variability in fetal size between individuals.
A second-trimester dating scan will generate or refine the due date, but with less precision than an early scan.
Due date versus gestational age — why the distinction matters
The due date is expressed as a single day, but pregnancy duration is more accurately expressed as a range: term pregnancy is defined as thirty-seven to forty-two weeks. A baby born at thirty-seven weeks is early term; at thirty-eight to forty weeks is full term; at forty-one weeks is late term; at forty-two weeks or beyond is post-term.
The due date of forty weeks sits in the middle of this range, not at its end. A pregnancy that continues to forty-one weeks is not overdue in a clinical sense — it is late term. Management of pregnancies that continue beyond forty-one or forty-two weeks depends on fetal wellbeing monitoring and clinical assessment, not simply on the distance from the due date.
What “gestational age” means on your ultrasound report
Ultrasound reports express the baby’s size in terms of gestational age: “fetal biometry consistent with X weeks and X days.” This is the age that corresponds to the baby’s measured size on average growth charts.
If the gestational age on an ultrasound report is different from what you calculated, this reflects one of two things:
Normal variation in fetal size: After the first trimester, babies vary in size between individuals — a baby who measures slightly larger or smaller than the average for their gestational age is most often simply larger or smaller, not more or less advanced in gestation. Size variation in the second and third trimesters does not routinely change the established due date.
A genuine dating discrepancy: If a large discrepancy exists between the ultrasound gestational age and the established due date — particularly if no first-trimester scan was done — your provider may revise the due date. This is more common when early scans were missed.
Gestational age in the Indian context
Several factors specific to the Indian population are worth noting in the context of fetal dating and growth assessment:
Reference charts: Most ultrasound machines use Western population growth reference charts. South Asian babies tend to be smaller on average than the Western babies on which many standard charts were developed. This means that a South Asian baby whose growth is entirely normal may be plotted below the fiftieth centile on a Western chart — which can cause unnecessary concern when it reflects normal population variation rather than growth restriction. Awareness of this limitation is worth raising with your provider if growth concerns are discussed.
PCOS and irregular cycles: The high prevalence of PCOS in Indian women makes LMP-based dating particularly unreliable in this population. First-trimester ultrasound dating is especially important for women with irregular cycles.
How to think about your due date as it approaches
As the due date approaches, it can become a source of increasing anxiety — particularly if family members, well-meaning friends, and daily awareness of the date combine to make each day after it feel like something is going wrong.
The more accurate way to think about it: your baby has a window of expected arrival, roughly from thirty-seven to forty-two weeks. Your due date is the midpoint of a normal range, not a deadline. A baby who arrives at forty-one weeks has not broken any rule. A baby who arrives at thirty-eight weeks has not arrived early — they are on time, by the definition of term.
Your provider will monitor you and your baby in the final weeks of pregnancy and will advise on timing of delivery if the pregnancy continues beyond forty-one to forty-two weeks. That monitoring — fetal movement, cardiotocography, ultrasound assessment of amniotic fluid and placental function — is what guides timing decisions, not the calendar proximity to the due date.
The honest message
Your due date is a useful estimate, not a prediction. It was calculated from the first day of your last period or from an early ultrasound, using methods that are good approximations but not precise forecasts. It may be revised as the pregnancy progresses. It is a midpoint in a range of normal, not an expiry date.
Know it. Use it for planning. Share it when people ask. And then hold it lightly — because your baby will arrive when they are ready, and the window of normal arrival is wider than the single date on your antenatal card suggests.
This article is for general educational purposes only and does not replace personalised medical advice. Always consult your doctor, midwife, or a qualified healthcare professional about your due date and gestational dating in your specific pregnancy.