Pregnancy & Due Date Calculator
Calculate your due date from your last menstrual period or ultrasound date. See how many weeks pregnant you are and estimate birth probability by week with our free online pregnancy calculator.
How to Use
- 1
Choose your calculation method: Last Menstrual Period (LMP) or Ultrasound
- 2
If using LMP: enter the first day of your last period and select your cycle length
- 3
If using Ultrasound: enter the scan date and gestational age at the time of scan
- 4
View your estimated due date, current week, trimester, and birth probability chart
- 5
Understanding the key inputs — Naegele's Rule adds 280 days (40 weeks) to the first day of your last period, assuming a 28-day cycle
- 6
Interpreting the results — the birth probability chart shows that most babies arrive between 37 and 42 weeks, with the due date being the middle of that window
Examples
Good Examples
Last period started January 1, 2026 (28-day cycle)
Due date: October 8, 2026 (280 days from LMP)Ultrasound at week 10 on March 15, 2026
Estimated LMP: January 3, 2026 → Due date: October 10, 2026Longer cycle adjustment
LMP with a 32-day cycle shifts the due date by 4 days later than a standard 28-day calculationTracking trimester progression
A due date of October 8, 2026 means the second trimester starts around April 2, 2026 (week 13)Planning maternity leave
With a due date of October 8, starting leave 2-4 weeks before gives a leave start date around September 10-24Bad Examples
Forgetting to adjust for cycle length
A 32-day cycle shifts the due date by ~4 days laterExpecting baby on exact due date
Only ~5% of babies arrive on the predicted due dateConfusing gestational age with fetal age
Gestational age counts from LMP (about 2 weeks before conception), so at "12 weeks pregnant" the baby is about 10 weeks oldCommon Mistakes
- Not adjusting for cycle length — Naegele's Rule assumes a 28-day cycle
- Confusing conception date with due date — conception occurs ~2 weeks after LMP
- Expecting the due date to be exact — babies arrive within a 2-week window most of the time
- Using ultrasound weeks as calendar weeks — ultrasound measures gestational age from estimated LMP
- Forgetting that irregular cycles make LMP-based calculations less accurate — ultrasound dating is more reliable in these cases
- Not realizing that IVF pregnancies use a different calculation method based on the embryo transfer date
- Assuming the first trimester starts at conception — it starts on the first day of your last period
Frequently Asked Questions
How is my due date calculated?
Your due date is calculated using Naegele's Rule: add 280 days (40 weeks) to the first day of your last menstrual period. If you know your cycle is longer or shorter than 28 days, the calculator adjusts accordingly.
How accurate is the due date?
Only about 5% of babies are born on their exact due date. Most babies arrive within a 2-week window before or after. The due date is best used as an estimate for planning purposes.
When does each trimester start?
First trimester: weeks 0–12, Second trimester: weeks 13–27, Third trimester: weeks 28–40+. Each trimester is roughly 13–14 weeks long.
Can I calculate my due date from an ultrasound?
Yes — if you know the date of your ultrasound and the gestational age measured at that scan, the calculator can work backwards to estimate your LMP and due date. This often yields a more accurate result than LMP alone.
How is pregnancy counted in weeks?
Pregnancy is counted from the first day of your last menstrual period (LMP), not from the date of conception. This means you are considered about 2 weeks pregnant at the time of actual conception. A full-term pregnancy is 40 weeks from LMP, which is roughly 38 weeks from conception.
What is the difference between gestational age and fetal age?
Gestational age counts from the first day of your last menstrual period and is typically 2 weeks more than fetal age. Fetal age (also called conceptional age) counts from the actual date of conception. Doctors typically use gestational age for consistency in medical care.
How accurate is Naegele's Rule for calculating due dates?
Naegele's Rule (add 280 days to LMP) assumes a 28-day cycle. It works well for women with regular cycles but can be off by several days for those with longer or shorter cycles. Only about 5% of babies arrive on their exact due date — most are born within a 2-week window.
How to calculate pregnancy weeks when cycles are irregular?
If your cycles are irregular, the LMP-based calculation may not be accurate. In this case, an early ultrasound (usually done at 8-12 weeks) provides the most reliable dating. Enter the ultrasound date and gestational age shown on the scan into the calculator for the best estimate.
When are each of the three trimesters and what happens in each?
First trimester (weeks 1-12): major organ development, morning sickness common. Second trimester (weeks 13-27): baby grows rapidly, energy usually returns. Third trimester (weeks 28-40): baby gains weight and prepares for birth. Each trimester is roughly 13-14 weeks long.