Calculators guide
How Pregnancy Due Dates Are Calculated, With Examples
· 4 min read
A due date is normally calculated by adding 280 days (40 weeks) to the first day of your last menstrual period, a method known as Naegele's rule. If your cycles are longer or shorter than 28 days, the date shifts by the difference. If you know the conception date or had an IVF transfer, a shorter, fixed number of days is counted from that date instead. Either way, the result is an estimate, not medical advice, and your care provider may adjust it after an ultrasound.
Why does pregnancy start before conception?
It surprises a lot of people that "week 1" of pregnancy is the week of your period, before you were pregnant at all. The reason is practical. Most people know when their last period started, but few know exactly when they ovulated. So pregnancy is dated from the last menstrual period (LMP), and conception is assumed to happen about two weeks later, in a typical 28-day cycle.
This is why someone who conceived four weeks ago is described as six weeks pregnant. It's also why the "how far along" figure on any due date tool is called gestational age.
The four ways to calculate it
The Pregnancy Due Date Calculator supports four starting points. Each one counts a set number of days to the due date:
| Calculate from | Days added | Why |
|---|---|---|
| First day of last period (28-day cycle) | 280 | 40 weeks from LMP |
| First day of last period (other cycle length) | 280 + (cycle − 28) | Ovulation shifts with cycle length |
| Conception date | 266 | 280 minus the 14 days before ovulation |
| IVF transfer, 3-day embryo | 263 | Embryo was already 3 days old |
| IVF transfer, 5-day embryo | 261 | Embryo was already 5 days old |
A worked example
Suppose your last period started on Saturday, August 1, 2026, your cycles are 28 days, and today is October 7, 2026. The calculator shows:
- Estimated due date: Saturday, May 8, 2027
- How far along: 9 weeks 4 days (first trimester)
- Days until due date: 213
- Estimated conception: Saturday, August 15, 2026
All three of the other methods land on the same day if they describe the same pregnancy. A conception date of August 15 gives May 8, 2027. A 5-day embryo transferred on August 20 also gives May 8, 2027.
What changes with a longer cycle?
With a 32-day cycle, ovulation tends to happen about four days later than in a 28-day cycle, so the due date moves four days later too. The same August 1 period gives a due date of Wednesday, May 12, 2027, and on October 7 you'd be 9 weeks 0 days, not 9 weeks 4 days. The calculator accepts cycle lengths from 20 to 45 days.
This adjustment assumes your cycles are fairly regular. If they vary a lot, the LMP method is less reliable and an early ultrasound becomes more important.
Reading the milestone table
Alongside the due date, you get a table of key dates. For the August 1 example:
| Milestone | Week | Date |
|---|---|---|
| End of first trimester | 13w 6d | Friday, November 6, 2026 |
| Anatomy scan window | 18–22 | December 5, 2026 – January 2, 2027 |
| Third trimester begins | 28 | Saturday, February 13, 2027 |
| Full term | 39 | Saturday, May 1, 2027 |
| Due date | 40 | Saturday, May 8, 2027 |
Two notes on how these weeks are written. "13w 6d" means 13 completed weeks and 6 days, the last day of the first trimester. And "full term" here marks 39 weeks, in line with the definition used by the American College of Obstetricians and Gynecologists, which describes 39 weeks 0 days to 40 weeks 6 days as full term.
How exact is a due date?
Not very, and that's normal. The calculator itself notes that only about 1 in 20 babies arrives on the due date. Most arrive within roughly two weeks on either side. A due date is best thought of as the middle of a window rather than a deadline.
Clinicians often confirm or adjust dating with a first-trimester ultrasound, which measures the embryo directly. If your provider gives you a different due date from a scan, use theirs. To see milestones based on their date, work backward: for example, choose Conception date and enter the day 266 days before the due date they gave you.
Using the calculator
- Under Calculate from, choose your starting point.
- Enter the Date. For the last-period method, also enter your Average cycle length (28 if you're not sure).
- Leave Today as is, or change it to see how far along you'll be on another date, such as an appointment.
- Read your due date, gestational age, trimester and the milestone table.
If the dates you enter would put the pregnancy in the future relative to "Today", or more than 44 weeks along, you'll see a message asking you to check them. The calculation happens in your browser, and nothing you enter is stored or sent.
FAQ
Can I use this before a positive test?
Yes. It only needs your last period date, but the result is only meaningful once a pregnancy is confirmed.
Why does my app show a different week?
Some apps count from conception or round to whole weeks. This calculator uses standard gestational age from the LMP, so check which convention your app uses.
What if I don't remember my last period?
Use a conception or transfer date if you have one. Otherwise, your provider can date the pregnancy by ultrasound.
To work out the fertile window before a pregnancy, see the Ovulation Calculator. To estimate your own dates, open the Pregnancy Due Date Calculator.