ReckonBoxHow the due date calculator works
An estimated due date is the first day of your last menstrual period plus 280 days, which is 40 weeks. Enter that date and the tool returns the due date, how far along you are today in the weeks-and-days form clinics use, and an estimated conception date. It also accepts a scan, an IVF transfer or a due date you already have.
Only about 4% to 5% of babies arrive on the estimated day. calculator.net puts it at 4% and the American Pregnancy Association at about 5%, which tells you something about the precision on offer. The date marks the middle of a window, and the tool prints that window: 37 to 42 weeks.
When a scan replaces the date from your last period
Published guidance has an ultrasound supersede the period-based estimate once the two disagree by more than a threshold that widens as pregnancy advances. Enter both a scan result and a last period here and the tool runs that check for you.
| Gestational age at the scan | Scan wins if the gap exceeds |
|---|---|
| Before 9 0/7 weeks | 5 days |
| 9 0/7 to 15 6/7 weeks | 7 days |
| 16 0/7 to 21 6/7 weeks | 10 days |
| 22 0/7 to 27 6/7 weeks | 14 days |
| 28 0/7 weeks and beyond | 21 days |
Perinatology.com implements this set, citing ACOG, AIUM and SMFM Committee Opinion 700 alongside Canadian guideline JOGC 388. It's the only calculator fetched for this build that does.
The mainstream tools stop short. calculator.net states its method as 40 weeks from the LMP with no redating rule at all. The NHS mentions that a 12-week scan can estimate the weeks more accurately, without saying it takes over. The American Pregnancy Association accepts an ultrasound input but publishes no threshold. ACOG's own page sits behind a paywall, returning 402 when fetched, so the guidance is genuinely hard for a pregnant person to reach.
Someone whose period-based date and 8-week scan differ by nine days has, until now, had nowhere consumer-facing to learn that the scan is the one that counts.
The 28-day assumption costs a week
Naegele's rule assumes a 28-day cycle with ovulation on day 14, so a longer cycle makes the standard calculation early. A 35-day cycle puts ovulation nearer day 21, which shifts the whole estimate about seven days later.
Seven days is not cosmetic. It's the difference between a conversation about post-dates induction happening and not happening.
The NHS calculator takes a cycle length. Perinatology takes one. Several consumer calculators never surface the field, quietly applying 28 to everybody. Omni is the only page fetched that writes the cycle-adjusted conception formula out in the open, as last period plus cycle length minus 14.
Dating gets less precise as pregnancy goes on
Crown-rump length in the first trimester is the most accurate measurement available, carrying a margin of about 5 days between 8 and 12 weeks, and some measurements land within 0 to 2 days of true gestational age.
After that it degrades steadily.
| Stage | Margin of error |
|---|---|
| First trimester (8 to 12 weeks, CRL) | About 5 days |
| 14 to 21 6/7 weeks | 7 to 10 days |
| 22 to 27 6/7 weeks | 10 to 14 days |
| After 28 weeks | 21 to 30 days |
Which is why the thresholds in the table above widen in step: a bigger discrepancy is needed to justify overriding, precisely because the scan itself is less certain.
There's a term for what happens when no scan ever lands. A pregnancy reaching 22 0/7 weeks without an ultrasound confirming or revising the due date is, in ACOG's phrase, suboptimally dated. This tool flags it when your dates are period-only past that point.
The other dating methods
Conception dating adds 266 days rather than 280, because conception happens roughly two weeks into a cycle. It's a window and not a moment: sperm survive up to seven days, so intercourse well before ovulation can still produce conception on ovulation day.
IVF is the precise one, because the embryo's age is known rather than inferred. A day-5 transfer counts 261 days to the due date and a day-3 transfer counts 263, so two transfers on the same calendar date land two days apart. Nothing is being guessed.
What this calculator does not do
Week-by-week fetal development content is absent. Every competing page bundles it, and it's editorial rather than calculation.
Nothing here indicates whether a pregnancy is progressing normally. A calculator cannot see that, and no arrangement of dates should be read as reassurance or alarm.
Gender prediction charts and similar are out, because they aren't calculation.
Results are estimates and not medical advice. A dating scan is the clinical standard, and the date in your notes belongs to your midwife or doctor. For the cycle side of the question, the ovulation calculator covers fertile windows and period timing, and the pregnancy weight gain calculator handles the Institute of Medicine ranges.
Frequently asked questions
How is a due date calculated? The standard method adds 280 days, or 40 weeks, to the first day of your last menstrual period. Dating from a known conception date adds 266 days instead, because conception happens roughly two weeks into a cycle. Both are estimates: calculator.net puts births on the estimated date at 4% and the American Pregnancy Association at about 5%.
When does an ultrasound replace the date from my last period? When the two disagree by more than a published threshold that widens as pregnancy advances. Perinatology.com, implementing ACOG, AIUM and SMFM Committee Opinion 700, gives the thresholds as more than 5 days before 9 0/7 weeks, 7 days at 9 0/7 to 15 6/7, 10 days at 16 0/7 to 21 6/7, 14 days at 22 0/7 to 27 6/7, and 21 days from 28 0/7 weeks on. Enter both a scan and a last period here and the tool runs that check.
Does my cycle length change my due date? Yes, and it is the most common reason a calculator is off. The standard rule assumes a 28 day cycle with ovulation on day 14, so a 35 day cycle pushes ovulation to roughly day 21 and moves the estimate about a week later. The NHS calculator and clinical tools take a cycle length; several consumer calculators do not surface one at all.
How accurate is pregnancy dating? Most accurate early and progressively worse later. Crown-rump length in the first trimester carries a margin of about 5 days between 8 and 12 weeks, with some measurements within 0 to 2 days. From 14 to 21 6/7 weeks the margin runs 7 to 10 days, from 22 to 27 6/7 it runs 10 to 14 days, and after 28 weeks it widens to 21 to 30 days.
What does suboptimally dated mean? It is the term for a pregnancy that reaches 22 0/7 weeks without an ultrasound confirming or revising the estimated due date. Dating rests on the last period alone at that point, which is the least precise basis available, and the scan that would have refined it is no longer able to do so with much precision.
How is a conception date estimated? By subtracting 266 days from the due date, or by adding cycle length minus 14 to the first day of the last period. It is a window, not a moment: sperm can survive up to 7 days, so intercourse several days before ovulation can result in conception on the ovulation day itself.
How does an IVF transfer change the calculation? IVF dating is more precise because the embryo age is known exactly. A day-5 transfer counts 261 days to the due date and a day-3 transfer counts 263, so two transfers on the same calendar date give due dates two days apart. Nothing is being estimated about when conception happened.
What counts as full term? Full term is 39 to 41 weeks. calculator.net sets out the surrounding categories as preterm before 37 weeks, early term 37 to 39, late term 41 to 42, and postterm after 42. The NHS describes a normal pregnancy as lasting 37 to 42 weeks from the first day of the last period.