Due Date by Ultrasound Calculator

Understanding the Due Date by Ultrasound Calculator

An ultrasound dating calculator works forward from the gestational age measured at a scan to estimate a due date. This is the more accurate method: current obstetric guidance gives first-trimester ultrasound precedence over dating from the last menstrual period.

How it actually works

Enter the gestational age measured at the scan in weeks and days, and the scan date. The calculator subtracts that gestational age from 280 days to find days remaining, and adds them to the scan date. A scan at 12 weeks 3 days on 15 June 2026 gives a due date of 25 December 2026.

The deeper context most people miss

Accuracy declines sharply as pregnancy progresses, and the reason is biological rather than technical: early embryos grow at a remarkably consistent rate, while later fetal growth varies substantially between individuals due to genetics, maternal factors, and placental function. This is why an early scan dates a pregnancy well and a late one dates it poorly.

Why crown-rump length is the most reliable measurement

Between roughly 8 and 13 weeks, the embryo is measured from the top of the head to the bottom of the buttocks, a dimension called crown-rump length, and it correlates with gestational age more tightly than any other measurement at any stage. The reason is that early growth is dominated by a genetically programmed developmental sequence that proceeds at a highly consistent rate across pregnancies, with relatively little influence from maternal nutrition, placental function, or fetal genetics for size. Growth divergence begins later, as those factors accumulate. From around 14 weeks, crown-rump length becomes impractical as the fetus curls and straightens, and dating switches to a composite of biparietal diameter, head circumference, abdominal circumference, and femur length, which together give a reasonable estimate but with wider error bounds because each measurement now reflects individual growth variation as well as age. By the third trimester, a fetus measuring large may be genuinely large rather than further along, and the two are difficult to distinguish, which is why third-trimester scans are generally not used to revise established dates. Current guidance including from ACOG and NICE is clear that dates established by an early ultrasound should not be changed by later scans, because a later measurement showing a discrepancy more likely reflects a growth issue than a dating error, and changing dates would obscure exactly the problem worth detecting.

A worked example: when the scan and the period disagree

Suppose someone reports a last menstrual period suggesting they are 13 weeks pregnant, but the scan measures 11 weeks 4 days. The discrepancy is about 10 days. Standard guidance is that in the first trimester, an ultrasound date differing from a menstrual date by more than about 5 to 7 days should replace it, so this pregnancy would be redated to the scan. The most likely explanation is that ovulation occurred later than day 14, which is extremely common, and the menstrual date was therefore too advanced. This matters practically because gestational age drives the timing of everything that follows. Combined first-trimester screening has an 11 to 14 week window, and being redated by 10 days could move someone from just inside to just outside it. Growth assessment later in pregnancy compares measurements against gestational age norms, so a pregnancy dated 10 days too advanced would show apparently poor growth throughout, potentially triggering unnecessary surveillance or intervention. And decisions about induction for post-term pregnancy are made against gestational age, so incorrect dating shifts the whole timeline. This is why establishing dates early and then holding them is standard practice, and why the redating threshold tightens as pregnancy advances: in the second trimester a larger discrepancy is required before dates are changed, and in the third trimester dates are generally not changed at all.

Deciding when dating matters most

Accurate dating matters throughout, and there are specific points where it is decisive. Screening windows are the most immediate: combined screening for chromosomal conditions requires 11 to 14 weeks, non-invasive prenatal testing needs sufficient fetal fraction from around 10 weeks, and the anomaly scan sits around 18 to 21 weeks. Each is invalid outside its window. Decisions at the threshold of viability, around 22 to 24 weeks, depend critically on gestational age, since management and counselling change substantially over a few days at that stage. Administration of corticosteroids for fetal lung maturation in threatened preterm birth is timed by gestation. Growth surveillance compares against gestational norms. Post-term management, where guidelines commonly recommend offering induction between 41 and 42 weeks given rising stillbirth risk, is triggered by dates. Given all this, anyone uncertain about their last menstrual period should say so explicitly rather than reporting an approximation, and should seek an early scan where available. Situations where menstrual dating is particularly unreliable include irregular cycles, conception soon after stopping hormonal contraception, conception while breastfeeding when ovulation can precede the return of periods, and early pregnancy bleeding mistaken for a period.

What else the dating scan does

A first-trimester scan performs several functions beyond dating, and knowing them helps set expectations. It confirms an intrauterine pregnancy, which excludes ectopic pregnancy, a potentially life-threatening condition where the pregnancy implants outside the uterus. It confirms viability by detecting fetal heart activity, which is present from around 6 weeks and whose absence at an expected stage indicates miscarriage. It determines the number of fetuses and, critically for twins, the chorionicity, meaning whether they share a placenta, which is best assessed in the first trimester and determines the entire subsequent management pathway since monochorionic twins carry substantially higher risks. It measures nuchal translucency between 11 and 14 weeks as part of combined screening for chromosomal conditions. It can detect some major structural abnormalities. And it identifies the location of the placenta and any uterine abnormalities. Because several of these are time-sensitive, particularly chorionicity determination and nuchal translucency, the timing of the first scan is chosen to serve multiple purposes rather than dating alone. It is also worth saying that scans occasionally reveal difficult findings, including missed miscarriage where a pregnancy has ended without symptoms, and attending prepared for that possibility is reasonable.

Variations: IVF dating, multiples, and late presentation

IVF pregnancies are dated exactly, since the date of fertilisation or transfer and the embryo's age at transfer are both known. Gestational age is calculated from those rather than from any measurement, and ultrasound is not used to revise it. This is the only situation where dating carries no meaningful uncertainty. Twin pregnancies are dated using the larger fetus in most protocols, since a size discrepancy more likely reflects a growth issue in the smaller twin than a dating difference. For women presenting late in pregnancy without earlier scans or reliable dates, dating uses second or third-trimester biometry with correspondingly wide error, and management proceeds with acknowledged uncertainty. Some protocols use transcerebellar diameter, which is relatively resistant to growth restriction, for late dating. Where dating remains genuinely uncertain, serial scans assessing growth velocity can help distinguish a small fetus from a younger one. Assisted conception other than IVF, including intrauterine insemination, has a known insemination date which improves dating over menstrual history.

Getting pregnancy dating right

Have a first-trimester scan where available, since crown-rump length between 8 and 13 weeks dates a pregnancy to within about 5 to 7 days, considerably better than menstrual dating. Accept the scan date over your calculated date if they differ by more than about a week in the first trimester, which is standard guidance and usually reflects later-than-assumed ovulation. Say explicitly if your cycles are irregular, if you conceived soon after stopping contraception or while breastfeeding, or if you had bleeding that might not have been a period, since all make menstrual dating unreliable. Understand that dates established early are not revised by later scans, because a later discrepancy more likely indicates a growth issue worth detecting than a dating error. And recognise that the dating scan also confirms location, viability, number, and chorionicity, several of which are time-sensitive.

What people get wrong

• Assuming a later scan can correct earlier dating, when dates established in the first trimester are deliberately not revised because a later discrepancy more likely reflects a growth issue.
• Relying on menstrual dating with irregular cycles, since ovulation timing is the main source of variability and can shift dates by a week or more.
• Treating third-trimester measurements as reliable for dating, when accuracy falls to around three weeks or more as individual growth variation dominates.
• Reporting an approximate last period date as though it were reliable, when saying the date is uncertain prompts an early scan that resolves it.

Where the math comes from

Days Remaining = 280 - (Gestational Weeks × 7 + Extra Days), and Due Date = Scan Date + Days Remaining. This works forward from the gestational age measured at the scan, using the standard 280-day pregnancy. Accuracy depends on when the scan was performed: crown-rump length between 8 and 13 weeks dates to within roughly 5 to 7 days, while third-trimester measurements carry errors of three weeks or more.

Questions and answers

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