Due Date Calculator: When Is My Baby Due and How Far Along Am I?
Enter the first day of your last period to get your estimated due date, how far along you are today, and the dates that count as full term.
Your estimated due date
- As of today
- Time to due date
- Estimated conception date
- Full-term window
How your due date is worked out
The calculator adds 280 days to the first day of your last period. That’s Naegele’s rule. It dates back to the 1800s, and obstetric guidelines still use it as the starting point.
The part that catches most people out: pregnancy weeks are counted from your period, not from conception. On the day you conceive, you’re already considered 14 days pregnant. That’s why a test you took “just last week” can come with the news that you’re five weeks along.
If your cycle isn’t 28 days
Naegele’s rule assumes a 28-day cycle. With a longer cycle you probably ovulated later, so the calculator pushes your due date back by the difference; with a shorter cycle it brings it forward. If your cycle falls outside 21–45 days, or changes a lot from month to month, rely on an ultrasound instead.
Your due date is really a due window
Very few babies arrive on their due date. A more useful number is the full-term window: babies born from the start of week 37 up to the start of week 42 are considered on time. The calculator shows this window for your dates.
For practical plans like packing your hospital bag, arranging leave or timing when family should fly in, plan around that window rather than a single day.
When an ultrasound changes your date
In the first trimester, the scan measures the baby from crown to rump. That measurement dates a pregnancy more accurately than a period date. Under the American College of Obstetricians and Gynecologists (ACOG) guidance, your due date is changed to match the scan when the two disagree by more than:
| When the scan is done | Due date changes if the difference is more than |
|---|---|
| Before 9+0 weeks | 5 days |
| From 9+0 up to 14+0 weeks | 7 days |
Once your date has been set by an early scan, it normally isn’t changed again. If a later scan says the baby is “measuring a week ahead,” that describes the baby’s size at that moment, not a new due date.
What US and UK guidance says
| US (ACOG) | UK (NICE) | |
|---|---|---|
| First estimate | Last period + 280 days | Period date taken at booking, then confirmed by scan |
| Dating scan | A first-trimester measurement is the most accurate method | A dating scan is offered to everyone in the first trimester |
| IVF | Dated from the transfer date and embryo age | — |
Starting prenatal care
In the US, call an OB-GYN, family physician or certified nurse-midwife once you have a positive test. Many offices schedule the first visit a few weeks later, and that visit or an early ultrasound will confirm your dates.
In the UK, contact your GP surgery or refer yourself to your local NHS maternity service. NICE recommends having the booking appointment by around 10 weeks, so get in touch early rather than waiting for your first scan letter.
Either way, bring the first day of your last period, your usual cycle length and a list of any medicines you take.
Frequently asked questions
How many weeks pregnant am I?
Count the days from the first day of your last period to today and divide by 7. The whole number is your week, the remainder is the extra days. The calculator above does this and also corrects for your cycle length.
Why is pregnancy counted from my last period and not from conception?
Most people don't know the day they conceived, but they usually remember when their last period started. So pregnancy is dated from that day everywhere in the world. Conception happens around two weeks later, which is why you are already 'a few weeks pregnant' when the test turns positive.
My periods are irregular. Can I still use this?
If your cycle length changes a lot from month to month, a date based on your last period can be misleading. An early ultrasound gives the most reliable date in that case, so treat the calculator's result as a rough guide.
Why doesn't my ultrasound due date match the calculator?
An ultrasound estimates gestational age by measuring the baby. A first-trimester measurement is more accurate than your period date, so if the two differ by more than a set margin, your provider will move your due date to match the scan. Once set this way, it usually stays the same for the rest of the pregnancy.
How is an IVF due date calculated?
With IVF the transfer date and the embryo's age are known exactly, so the date is calculated from those rather than from a period. Use our IVF due date calculator for that.
What happens if I go past my due date?
Going a few days over is common and not a problem in itself. You'll usually be checked more often, and if the pregnancy goes beyond a certain point your provider may recommend induction. If you notice your baby moving less than usual, call your maternity unit or provider right away instead of waiting for your next appointment.
About this page
- Medical review
- Emergency medicine physician · October 24, 2026
- Last updated
Sources
- Methods for Estimating the Due Date. Committee Opinion No. 700. American College of Obstetricians and Gynecologists (ACOG), 2017.
- Definition of Term Pregnancy. Committee Opinion No. 579. American College of Obstetricians and Gynecologists (ACOG), 2013.
- Antenatal care. NICE guideline NG201. National Institute for Health and Care Excellence (NICE), 2021.
This page is for general information and does not replace an examination or diagnosis.