The due date calculator estimates your pregnancy due date using Naegele's rule from your last menstrual period (LMP) or from your conception date. It shows your current gestational age, trimester, and key milestones.
Calculate Your Due Date
Naegele's rule: LMP + 280 days (40 weeks)
Due date = conception date + 266 days (38 weeks)
Wk 1–12
Wk 13–27
Wk 28–40
Key Milestones
How to Calculate Your Due Date
Calculating a pregnancy due date is one of the first things expectant parents want to know. The standard method, called Naegele's rule, gives a reliable estimate that your healthcare provider will use until an ultrasound can provide a more precise date.
The LMP method (Naegele's Rule)
The most common method uses the first day of your last menstrual period (LMP). Add 280 days (40 weeks) to that date. For example, if your LMP was January 1, your estimated due date is October 8. The calculation assumes a regular 28-day cycle with ovulation on day 14. If your cycles are longer or shorter, your provider may adjust the date.
The conception method
If you know your exact conception date — for example through IVF or careful tracking — use the conception method instead. Add 266 days (38 weeks) from conception. This is equivalent to the LMP method because conception typically occurs about 14 days after your period starts. For IVF, count from the egg retrieval or embryo transfer date, not the LMP.
Understanding trimester divisions
Pregnancy is divided into three trimesters. The first trimester (weeks 1-12) covers early development when the risk of miscarriage is highest. The second trimester (weeks 13-27) is often called the "honeymoon phase" — many symptoms ease and the pregnancy becomes visible. The third trimester (weeks 28-40) is the final growth phase before birth.
When will your provider confirm the date?
Your first prenatal ultrasound (usually between 8-14 weeks) is used to confirm or adjust the due date based on the baby's size. Ultrasound dating is most accurate in the first trimester. After 20 weeks, ultrasound dating becomes less reliable. Your provider may give you a date range rather than a single day.
FAQ
Is this due date calculator free?
Yes, completely free with no signup required. All calculations happen in your browser.
How accurate is the estimated due date?
The LMP (last menstrual period) method uses Naegele's rule, which assumes a 28-day cycle with ovulation on day 14. Only about 5% of babies are born exactly on their due date — most arrive within two weeks before or after. Your healthcare provider may adjust the date based on ultrasound measurements.
What is Naegele's rule?
Naegele's rule is the standard method for calculating due dates from the last menstrual period: take the first day of your LMP, add one year, subtract three months, and add seven days. This equals LMP + 280 days (40 weeks). It was developed by German obstetrician Franz Naegele in the early 1800s and remains the standard today.
What is gestational age?
Gestational age is the number of weeks and days since the first day of your last menstrual period — even before conception occurred. At conception, you're already counted as 2 weeks pregnant. A full-term pregnancy is typically 37-40 weeks gestational age.
What is the difference between LMP and conception date?
LMP (last menstrual period) date is used for the standard 40-week calculation. If you know your actual conception date (e.g., from IVF), use the conception method — it adds 266 days (38 weeks) from conception, which equals the same final due date because conception typically occurs ~14 days after LMP.
When does each trimester begin?
First trimester: weeks 1-12. Second trimester: weeks 13-27. Third trimester: weeks 28-40. Some providers define second trimester as starting at 14 weeks — there is slight variation by source.