Pregnancy Due Date Calculator
Estimate your baby’s due date, pregnancy week, trimester and important pregnancy milestones using your last menstrual period or conception date.
Calculate Your Estimated Due Date
Your Pregnancy Estimate
How the Pregnancy Due Date Calculator Works
A pregnancy due date is an estimate of when a baby may be born. The standard calculation traditionally uses the first day of the last menstrual period (LMP) and estimates a pregnancy length of about 40 weeks, or 280 days, from that date.
If you know the conception date instead, this calculator uses approximately 266 days from conception. If your average menstrual cycle is longer or shorter than 28 days, the LMP estimate is adjusted by the difference in cycle length.
Estimated due date = LMP + 280 days + (cycle length − 28 days)Estimated due date = conception date + 266 daysThe result is an estimate, not a guarantee of the exact birth date. Pregnancy length can vary considerably, and your healthcare professional may revise the estimated due date based on clinical information or ultrasound findings.
Pregnancy Milestones by Week
| Pregnancy stage | Approximate weeks | What it means |
|---|---|---|
| First trimester | Weeks 1–13 | Early pregnancy development and major fetal development. |
| Second trimester | Weeks 14–27 | Growth continues and many pregnancy symptoms change. |
| Third trimester | Weeks 28–40+ | Rapid growth and preparation for birth. |
| Estimated due date | Around 40 weeks | The conventional estimated date used for pregnancy dating. |
Pregnancy weeks are generally counted from the first day of the last menstrual period, which is about two weeks before conception in a typical 28-day cycle.
Why Your Due Date May Change
Your calculated due date is a useful starting estimate, but the actual date of birth may be earlier or later. Ovulation does not always occur on the same day in every cycle, and the timing of implantation and conception can vary.
Healthcare professionals may use menstrual history, physical examination, ultrasound and other clinical information to establish or adjust pregnancy dating. An ultrasound can be particularly useful when the menstrual dates are uncertain or cycles are irregular.
What if my periods are irregular?
If your cycles are irregular, an LMP-only calculation may be less precise because ovulation may not have occurred around the usual midpoint of the cycle. Discuss your dates and any ultrasound dating with your healthcare professional.
Can a baby arrive before or after the due date?
Yes. A due date is an estimate rather than a deadline. Only a relatively small proportion of babies are born exactly on their estimated due date.
How to Use This Due Date Calculator
- Select whether you want to calculate from your last menstrual period or conception date.
- Enter the relevant date.
- If using LMP, select your usual average menstrual cycle length.
- Click Calculate Due Date.
- Review the estimated due date, pregnancy age, trimester and milestone dates.
Frequently Asked Questions
How accurate is a pregnancy due date calculator?
A due date calculator provides an estimate. Accuracy depends on how accurately the pregnancy start date is known and whether ovulation occurred at the expected time. Clinical assessment and ultrasound may provide a more appropriate pregnancy date when dates are uncertain.
How many weeks is a normal pregnancy?
Pregnancy is conventionally dated as about 40 weeks from the first day of the last menstrual period. The actual duration varies between pregnancies.
Why are 40 weeks counted from my last period?
Pregnancy dating traditionally starts on the first day of the last menstrual period because that date is often easier to identify than the exact date of ovulation or conception.
What if I know my conception date?
If you know the conception date, the calculator estimates the due date using approximately 266 days from conception.
Does cycle length affect my due date?
It can. The traditional LMP calculation assumes a 28-day cycle. This calculator adjusts the estimate by the difference between your selected average cycle length and 28 days.
What is gestational age?
Gestational age is the age of the pregnancy, conventionally measured from the first day of the last menstrual period rather than from conception.
Can an ultrasound change my due date?
Yes. Depending on the circumstances and stage of pregnancy, a healthcare professional may use ultrasound measurements and other clinical information to assess pregnancy dating.
What happens if I do not remember my last period?
If your LMP is unknown or uncertain, tell your healthcare professional. Other information, including ultrasound dating, may help establish an estimated due date.
Is the due date the day my baby will be born?
No. It is an estimated date. Birth can occur before or after the estimated due date.
Can I use this calculator for twins?
The date calculation can estimate a pregnancy due date, but pregnancies with multiples have different clinical considerations. Your healthcare team should provide individualized guidance.
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Disclaimer
This calculator is provided for educational and informational purposes only. It does not provide medical advice, diagnosis or treatment. Do not use the calculated due date to make medical decisions without consulting a qualified healthcare professional. If you have concerns about your pregnancy, contact your doctor, midwife or other appropriate healthcare provider.