Pregnancy Due Date Calculator
Work out your estimated due date from your last period, the date of conception, or an IVF transfer date. You will also see your current gestational age in weeks and days, which trimester you are in, and the dates of the key milestones along the way.
Explain like I'm 5 (how the date is worked out)
A human pregnancy is counted as 40 weeks long, measured from the first day of your last period. That is a slightly odd convention (you are not actually pregnant for the first two weeks of it), but it is the one every midwife and scan department uses. Add 280 days to the first day of your last period and you get your due date. The other methods (conception, IVF transfer) are just adjusted versions of the same sum.
Calculate
Enter a date and press Calculate.
Your pregnancy at a glance
- Estimated due date—
- Current gestational age—
- Current trimester—
Key milestone dates
- End of first trimester (13 weeks)—
- Anatomy scan window (18 to 22 weeks)—
- Viability (24 weeks)—
- Glucose screening window (24 to 28 weeks)—
- End of second trimester (27 weeks)—
- Full term (37 weeks)—
- Due date (40 weeks)—
- Late term (41 weeks)—
Prove it
Naegele's rule: estimated due date = LMP + 280 days. Conception-based: conception date + 266 days. IVF 5-day (blastocyst) transfer: transfer date + 261 days. IVF 3-day transfer: transfer date + 263 days. Gestational age is the time elapsed since the LMP-equivalent date (due date minus 280 days).
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How the due date is worked out
The standard method is called Naegele's rule, named after the nineteenth-century German obstetrician Franz Naegele. You take the first day of the last menstrual period and add 280 days (40 weeks). The reasoning is that ovulation tends to happen roughly 14 days into a typical 28-day cycle, and a pregnancy runs for about 38 weeks from conception, so 14 + 266 = 280 days from the start of the last period.
If you know your conception date (for example, if you were tracking ovulation), the sum becomes conception + 266 days. For IVF, the transfer date is a known point in embryonic development, so a 5-day blastocyst transfer counts as the embryo already being 5 days old, and you add 261 days. A 3-day transfer adds 263. Either way, you end up at the same 280-day gestational age by the time the baby arrives.
How accurate is any of this?
Due dates are a best guess, not a booking. Only about one baby in twenty actually arrives on the estimated date. Most turn up within two weeks either side, which is why obstetric services treat 37 to 42 weeks as the normal range rather than singling out week 40 as the target. If your cycle is reliably 28 days, Naegele's rule is a reasonable first pass. If your cycle is irregular, or you are not sure when your last period started, the dating ultrasound your midwife arranges in the first trimester will be more accurate than any calculation. That scan measures the embryo directly, and before about 13 weeks that measurement is the most reliable dating method available.
For IVF pregnancies the transfer date is known to the day, so the calculated date is usually spot on. The only uncertainty is still the delivery itself, which is influenced by everything from genetics to whether it is your first baby (firstborns tend to run a few days late on average).
Gestational age, foetal age, and a short glossary
Clinicians talk about pregnancies in weeks and days, not months. A few terms worth knowing:
- Gestational age: weeks counted from the first day of your last period. This is the number your midwife will use.
- Foetal age (or embryonic age): counted from conception, so roughly two weeks less than gestational age.
- First trimester: weeks 1 to 13 (some services define it as ending at week 12).
- Second trimester: weeks 14 to 27.
- Third trimester: week 28 onwards.
- Viability: around 24 weeks. Before this, survival outside the womb is very unlikely. After 24 weeks it rises sharply with each additional week.
- Early term: 37 to 38 weeks and 6 days.
- Full term: 39 to 40 weeks and 6 days.
- Late term: 41 to 41 weeks and 6 days.
- Post-term: 42 weeks and beyond.
What the milestone dates are for
The milestone dates in the results give you a rough map of the pregnancy: when booking bloods and the dating scan tend to happen, when the anatomy (20-week) scan is done, when the baby reaches the point of likely survival if born early, when gestational diabetes screening is usually offered, and when you cross into full term. Your actual appointments will be set by your midwife and may fall slightly outside these windows depending on local NHS or private-care protocols, but the ranges here are the standard ones.
A necessary note
This calculator is informational. It does not replace antenatal care, a dating scan, or a conversation with your midwife or GP. If you are pregnant, or think you might be, get yourself booked into maternity care properly. That is where the real monitoring happens.
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Frequently asked questions
How accurate is a due date calculator?
Only about 5% of babies arrive on the exact due date. Most turn up within two weeks either side, which is why the obstetric normal range is 37 to 42 weeks rather than a single day. A first-trimester dating scan is more accurate than any calculation, especially if your cycle is irregular.
What if my cycle is not 28 days?
Naegele's rule assumes a 28-day cycle with ovulation on day 14. Longer cycles shift the date later; shorter cycles shift it earlier. Roughly, add a day to the due date for every day your cycle is longer than 28, and subtract one for every day shorter. Your dating scan will correct for any mismatch.
Can I use this after IVF?
Yes. Pick the IVF option and enter your transfer date. The calculator adjusts for a 3-day or 5-day (blastocyst) transfer, because those are different points in embryonic development. IVF dates tend to be the most accurate of the three methods because the transfer date is known precisely.
When does each trimester start and end?
First trimester: weeks 1 to 13. Second trimester: weeks 14 to 27. Third trimester: week 28 to birth. Full term starts at 37 weeks, late term at 41, and anything beyond 42 is considered post-term.
Gestational age versus foetal age, what's the difference?
Gestational age is counted from the first day of your last period. Foetal age is counted from conception, which is about two weeks later. A 10-week gestational pregnancy is about 8 weeks by foetal age. Clinicians almost always use gestational age, so that is the one on your notes.