Due Date: 40 Weeks From LMP or 38 Weeks From Conception?

Is My Due Date 40 Weeks From My Last Period or 38 Weeks From Conception?

Many women ask the same question: is due date 40 weeks from LMP or conception? The answer is both. The two counts give the same due date. They simply start from two different days.

Pregnancy is counted as 40 weeks, or 280 days, from the first day of your last period. It is also counted as 38 weeks, or 266 days, from conception. The gap between the counts is exactly two weeks. Those two weeks are the time before conception happens.

This article shows both calculations with real dates. It also explains which date you should use. In addition, it covers longer and shorter cycles. Finally, it explains what happens if you do not know your last period.

Use the free Due Date Calculator on pregfitcalc.com to check your own date. You can enter a last period date or a conception date. The calculator then shows your due date, your gestational age and your trimester.

A due date is an estimate, not a promise. Only a small number of babies arrive on the exact date. Most babies arrive in the weeks around it. Your doctor or midwife confirms your official date, usually with an early scan.

Due date chart showing 40 weeks from the last period and 38 weeks from conception giving the same due date

How to Use the Due Date Calculator

  1. First, choose your method. Pick “Last Period” if you know the first day of your last period. Pick “Conception” if you know the day you conceived.
  2. Next, enter your date. Use the exact first day of your last period, not the day it ended. For conception, use your ovulation or conception day.
  3. Then, add your cycle length if the calculator asks for it. The standard cycle is 28 days. A longer cycle moves your due date later. A shorter cycle moves it earlier.
  4. Finally, read your result. The calculator shows your estimated due date. It also shows how far along you are today. Compare it with the date from your early scan later.

The Short Answer: Both Counts Give the Same Date

The last period count starts about two weeks before conception. Conception usually happens near ovulation. Ovulation falls around day 14 of a 28-day cycle. So, the period count starts roughly 14 days earlier.

Because it starts earlier, the period count needs more days. It needs 280 days in total. The conception count starts later, so it needs fewer days. It needs only 266 days.

280 days minus 266 days is 14 days. That is exactly the two-week head start. In short, the two methods describe the same pregnancy. They just use different starting lines.

Why Does the Count Start Before the Baby Exists?

This is the part that confuses almost everyone. At conception, you are already counted as two weeks pregnant. The baby is not there yet for those first two weeks. Still, the medical count includes them.

Doctors use this system for a practical reason. Most women know the date of their last period. Very few women know their exact conception day. A shared counting system also helps every clinic. Appointments, scans and tests are then planned the same way everywhere.

This count is called gestational age. It is always about two weeks ahead of the baby’s real age. The baby’s real age from conception is called fetal age. So, at 12 weeks pregnant, the baby is about 10 weeks old.

Worked Example: One Due Date, Two Routes

Take a last period that started on 1 March 2026. Add 280 days. The due date is 6 December 2026.

Now take the conception route for the same pregnancy. With a 28-day cycle, conception falls around 15 March 2026. Add 266 days to that date. The due date is again 6 December 2026.

| Method | Start date | Days to add | Due date |
| From last period | 1 March 2026 | 280 days (40 weeks) | 6 December 2026 |
| From conception | 15 March 2026 | 266 days (38 weeks) | 6 December 2026 |

As the table shows, both routes land on the same day. Neither route is wrong. They are two ways of counting one pregnancy.

Which Date Should You Use?

Use your last period if you know its first day. Also use it if your cycles are fairly regular. This is the method most calculators use first. It is also the method most clinics start with.

Use your conception date if you know it well. For example, you may have tracked ovulation closely. IVF patients also know their exact dates. In those cases, the conception route is very precise.

However, most “known” conception dates are really estimates. A home ovulation test shows a hormone surge, not the exact hour. Sperm can also survive for up to five days. So, the conception day can shift by a few days. That is why clinics confirm dates with a scan.

What If Your Cycle Is Longer or Shorter Than 28 Days?

The 280-day rule assumes a 28-day cycle. It also assumes ovulation on day 14. A longer cycle usually means later ovulation. A shorter cycle usually means earlier ovulation.

The fix is simple. Add the difference to your due date. First, subtract 28 from your cycle length. Then, add that many days to the standard due date.

For example, take a 35-day cycle. 35 minus 28 is 7. So, the due date moves 7 days later. With a last period of 1 March 2026, the due date becomes 13 December 2026, not 6 December.

In contrast, a 24-day cycle moves the date earlier. 28 minus 24 is 4, so the date moves about 4 days earlier. If your cycles are very irregular, neither method is reliable alone. In that case, an early scan matters much more.

What If You Do Not Know Your Last Period?

Do not guess wildly. Many women cannot remember the exact date. Cycles can also be irregular after birth control or breastfeeding. In these cases, doctors rely on an early ultrasound scan.

A first-trimester scan measures the baby directly. It is the most accurate way to confirm dating at that stage. Your provider may then set your official due date from the scan. Once that date is set, later scans check growth instead. They do not keep moving the date.

If you have no dates at all, book your first appointment early. Tell the clinic what you do and do not know. They will arrange a dating scan for you.

Frequently Asked Questions

Is pregnancy 38 weeks or 40 weeks long?


Both numbers are correct. Pregnancy lasts about 40 weeks from the last period. It lasts about 38 weeks from conception. The difference is the two weeks before conception.

Why was I already 4 weeks pregnant at my positive test?


A test usually turns positive around a missed period. By then, about four weeks have passed since your last period started. Conception happened about two weeks before the test. So, you are counted as four weeks pregnant, although the baby is about two weeks old.

Which is more accurate, the last period or the conception date?


An exactly known conception date is more precise. However, conception is usually estimated, not known. That is why clinics start with the last period and confirm with an early scan.

My cycle is 35 days. Which due date is right for me?


Your ovulation probably happens around day 21, not day 14. So, add about 7 days to the standard 40-week date. A good calculator does this when you enter your cycle length.

Can my due date change after a scan?


Yes, it can change early in pregnancy. If the scan date differs a lot from the period date, your provider may use the scan date. After that, the date usually stays fixed. Later scans then measure growth, not dates.

Is fetal age the same as gestational age?


No. Gestational age counts from the last period. Fetal age counts from conception. Fetal age is about two weeks less than gestational age.

Related Tools on pregfitcalc.com

Sources

Disclaimer

This article and calculator give general information only. They do not replace your doctor’s or midwife’s advice. Always confirm your due date and your care plan with your own care team