Pregnancy Due Date Calculator by Last Period & Conception Date
Find out when your baby is due with this free pregnancy due date calculator. Enter the first day of your last menstrual period (LMP) and your average cycle length, or use your known conception date instead. You will get your estimated due date, how many weeks and days pregnant you are today, your current trimester, your likely conception window, and the key milestone dates of your pregnancy.
The calculation follows Naegele’s rule, the standard method recommended by the American College of Obstetricians and Gynecologists (ACOG): add 280 days (40 weeks) to the first day of your last period. If your cycle is longer or shorter than 28 days, the due date is adjusted by the same number of days, because ovulation — and therefore conception — happens later or earlier.
Your Estimated Due Date
How Far Along You Are
Trimester
Likely Conception Window
Ovulation usually happens about 14 days before your next period.
Days Until Due Date
Key Milestone Dates
Milestone
Date
End of 1st trimester (13 weeks, 6 days)
Start of 2nd trimester (14 weeks)
End of 2nd trimester (27 weeks, 6 days)
Start of 3rd trimester (28 weeks)
Early term begins (37 weeks)
Full term begins (39 weeks)
Important: Your due date is an estimate, not a prediction. Only about 4–5% of babies are born on their exact due date, and most arrive within two weeks of it. A first-trimester ultrasound is the most accurate way to date a pregnancy — only a clinician or ultrasound can confirm your due date.
How to Use This Pregnancy Due Date Calculator
Choose your method. Use “By Last Period” if you know the first day of your last menstrual period, or “By Conception Date” if you know the day you conceived (for example, after tracking ovulation or fertility treatment).
Enter your date. Pick the first day of your last period from the calendar, or enter your conception date.
Set your cycle length (LMP method only). Leave it at 28 days if your cycles are regular and about 28 days long. If your cycles are usually longer, enter your average length — the due date moves later by the same number of extra days. If they are shorter, it moves earlier.
Click “Calculate My Due Date.” Your estimated due date, current week and day, trimester, conception window and milestone dates appear instantly.
Share the result with your clinician at your first prenatal appointment so they can confirm the date with an ultrasound.
How the Due Date Is Calculated
The calculator uses Naegele’s rule, the method described in ACOG Committee Opinion No. 700, Methods for Estimating the Due Date (2017, reaffirmed 2025):
From your last period: Estimated due date = first day of LMP + 280 days + (your cycle length − 28 days). The 280 days equal 40 weeks, the length of a typical pregnancy counted from the last period.
From conception: Estimated due date = conception date + 266 days (38 weeks). Conception usually happens about 14 days after the first day of the last period in a 28-day cycle, so 266 days from conception equals 280 days from the last period.
Gestational age is counted from the first day of your last period, not from conception — so on the day you conceive, you are already counted as about two weeks pregnant.
Trimesters (ACOG): 1st trimester — first day of LMP through 13 weeks and 6 days; 2nd trimester — 14 weeks and 0 days through 27 weeks and 6 days; 3rd trimester — 28 weeks and 0 days through 40 weeks and 6 days.
Term definitions (ACOG Committee Opinion No. 579): early term is 37 weeks 0 days through 38 weeks 6 days, full term is 39 weeks 0 days through 40 weeks 6 days, late term is 41 weeks, and post-term is 42 weeks or later.
Worked Example
Example: Your last period started on January 1, 2026 and your average cycle is 28 days.
Estimated due date: Thursday, October 8, 2026 (January 1, 2026 + 280 days).
Likely conception: around January 15, 2026 (about 14 days after the last period began).
End of 1st trimester: April 8, 2026 (13 weeks, 6 days).
Start of 2nd trimester: April 9, 2026 (14 weeks).
End of 2nd trimester: July 15, 2026 (27 weeks, 6 days).
Start of 3rd trimester: July 16, 2026 (28 weeks).
Full term begins: October 1, 2026 (39 weeks).
If the same person had a 32-day cycle instead, the due date would move 4 days later, to October 12, 2026 — because ovulation and conception would likely have happened 4 days later.
Frequently Asked Questions
How accurate is a due date calculated from my last period? It is a good estimate for planning, but not exact. A first-trimester ultrasound is the most accurate dating method and is accurate to within about 5–7 days. ACOG recommends changing the due date to match the ultrasound when the two differ by more than a set amount. Only about 4–5% of babies are born on their exact due date.
Why is pregnancy counted from my last period instead of conception? Most people know the date of their last period, but the exact day of conception is often unknown. By convention, pregnancy is dated from the first day of the last menstrual period, which is usually about two weeks before conception in a 28-day cycle. That is why a “40-week” pregnancy is counted from the last period, even though the baby has been developing for about 38 weeks.
What if my cycles are irregular or longer than 28 days? Enter your average cycle length and the calculator adjusts the due date: a longer cycle moves the due date later by the extra days, a shorter cycle moves it earlier. If your cycles are very irregular, or you do not remember your last period, an early ultrasound is the best way to date the pregnancy — ask your clinician.
Can I use the conception date instead? Yes. If you know the day you conceived — for example, because you tracked ovulation or had fertility treatment — choose the “By Conception Date” tab. The calculator adds 266 days (38 weeks) to the conception date, which gives the same result as the last-period method for a typical 28-day cycle.
What are the trimesters, and when do they change? Following ACOG, the 1st trimester runs from the first day of your last period through 13 weeks and 6 days, the 2nd trimester from 14 weeks through 27 weeks and 6 days, and the 3rd trimester from 28 weeks until birth. Full term is 39 weeks through 40 weeks and 6 days.
What if my due date has already passed? The calculator will show that your due date has passed and how many days ago it was. If you are past 40 weeks, contact your clinician — pregnancies at 41 weeks are called late term and at 42 weeks or later post-term, and your clinician will usually want to monitor you closely and discuss next steps.
Disclaimer: This calculator provides estimates for general information only and is not medical advice. It does not diagnose pregnancy, confirm a due date, or replace prenatal care. Always confirm your due date and pregnancy dating with a qualified healthcare professional, ideally with a first-trimester ultrasound. If you have bleeding, severe pain, reduced fetal movement, or any other worrying symptom, seek medical care promptly. No information you enter is saved, stored, or sent anywhere — all calculations happen in your browser.
Current as of November 2026. Figures follow ACOG Committee Opinion No. 700 (Methods for Estimating the Due Date, 2017, reaffirmed 2025) and ACOG Committee Opinion No. 579 (Definition of Term Pregnancy, 2013).