How to Calculate Your Due Date

What You Need Before You Start

Calculating a due date is really just choosing the right starting point and adding a fixed number of days. Before you begin, gather whichever of these you have — the more anchors, the more confident your estimate.

• First day of your last menstrual period (LMP). This is the single most common starting point, and the one your doctor will ask for first.
• Your average cycle length. Needed only if your cycles are not 28 days — most due date tools adjust for this automatically.
• Your ovulation or conception date, if you tracked it with ovulation tests, basal body temperature, or fertility charting.
• Your embryo transfer date and embryo age (Day 3, Day 5, or Day 6) if this is an IVF pregnancy.
• An early ultrasound result, ideally between 8 and 14 weeks, which gives the most reliable measurement for natural pregnancies.

Method 1: From Your Last Period (LMP)

This is the method your doctor uses first and the one behind almost every due date calculator online. It is based on Naegele's Rule: human pregnancy lasts about 280 days (40 weeks) measured from the first day of the last menstrual period.

The LMP formula

Worked example

Say the first day of your last period was January 1, 2026 and your cycles are 28 days:

1. Write down January 1, 2026.
2. Add 280 days → October 8, 2026.
3. No cycle adjustment needed (28-day cycle).

Your estimated due date is Thursday, October 8, 2026. You can double-check with the classic Naegele arithmetic: January 1, 2026, plus one year is January 1, 2027; minus three months is October 1, 2026; plus seven days is October 8, 2026. Same answer.

Try the LMP due date calculator →

Method 2: From Your Conception Date

If you know when conception happened — for example, you used ovulation tests or tracked your fertile window — you can skip the period math entirely. Pregnancy lasts about 266 days (38 weeks) from conception, because conception typically occurs around two weeks after the LMP.

The conception formula

Worked example

Say you know you conceived on February 14, 2026 (ovulation confirmed by test):

1. Write down February 14, 2026.
2. Add 266 days → November 7, 2026.

Your estimated due date is Saturday, November 7, 2026. This method is especially useful when your cycles are irregular, because it does not assume ovulation happened on day 14.

Try the conception due date calculator →

Method 3: From an Ultrasound Scan

An early ultrasound measures your baby directly and works backward to your due date. Between 8 and 14 weeks, the crown-rump length (CRL) measurement is accurate to about ±5 days — better than period math for most people.

The ultrasound formula

Worked example

Say you had a scan on October 5, 2026 and the sonographer measured you at 12 weeks 0 days (84 days):

1. Write down October 5, 2026.
2. Subtract the gestational age from 280: 280 − 84 = 196 days remaining.
3. Add 196 days to the scan date → April 19, 2027.

Your estimated due date is Monday, April 19, 2027. One caution: later scans are less precise for dating (±2–3 weeks at 20 weeks), so the earliest scan you have is the one that counts most.

Try the ultrasound due date calculator →

Method 4: After IVF or Embryo Transfer

IVF dating is the most precise of all, because the embryo's exact age at transfer is known — there is no guessing about when ovulation happened. Your clinic will have already calculated this, but it is easy to verify yourself.

The IVF formulas

Worked example

Say you had a Day 5 transfer on September 1, 2026:

1. Write down September 1, 2026.
2. Add 261 days → May 20, 2027.

Your estimated due date is Thursday, May 20, 2027, accurate to about ±1–3 days. Frozen embryo transfers (FET) use exactly the same math — freezing does not change the embryo's age.

Try the IVF due date calculator →

Adjusting for Your Cycle Length

The standard 280-day count quietly assumes a 28-day cycle with ovulation on day 14. But cycles commonly run 21 to 35 days, and ovulation usually happens about 14 days before your next period — not 14 days after your last one. That detail is what the adjustment corrects.

For a 35-day cycle, ovulation typically lands around day 21, a full week later than the textbook day 14. So everything shifts a week later: EDD = LMP + 280 + 7 days. With an LMP of January 1, 2026, a 35-day cycle gives Thursday, October 15, 2026 instead of October 8.

The reverse applies to short cycles. With a 21-day cycle, ovulation comes around day 7, so EDD = LMP + 280 − 7 days. An LMP of January 1, 2026 gives Thursday, October 1, 2026.

If you know your actual ovulation day from tracking, that beats cycle-length math every time — just treat it as your conception date and add 266 days.

Which Method Should You Trust?

No single method is perfect, but they are not equal either. LMP math is fast and free, and it is right for most people with regular cycles — but it assumes things about your ovulation that may not be true. A first-trimester ultrasound measures reality instead of assumptions, which is why it wins when the two disagree by more than a week.

Once you have an estimate, follow your baby's progress week by week in our pregnancy week-by-week guide — it is indexed to the same gestational-age counting your due date uses, so week 12 in the guide means 12 weeks from your LMP, exactly as calculated above.

One last piece of honesty: a due date is an estimate with a margin of error, not a delivery appointment. Only about 5% of babies arrive on the exact date. If your calculation worries you — an unknown LMP, very irregular cycles, or methods that disagree wildly — bring it to your first prenatal visit. Sorting out dating early is one of the most useful things that appointment does.