π€° Due Date Calculator
Your due date, weeks pregnant, and trimester timeline.
Estimated Due Date
Estimates only, not medical advice. Your clinician may adjust dating with ultrasound. Seek prenatal care early.
How to Use This Calculator
Choose whether you know your last period date or your conception date, enter it, and the calculator returns your due date, how many weeks pregnant you are today, and your trimester timeline.
Select "First day of last period" (most common) or "Conception date" if you know it precisely, such as with IVF or careful tracking.
Enter the date. For LMP, use the first day of full flow, not spotting.
Read your due date and current week. The trimester table shows the date range for each trimester.
Compare with your clinician's dating at your first prenatal visit, since ultrasound can shift the estimate by a few days.
Naegele's Rule
Naegele's rule assumes a 28-day cycle with ovulation on day 14, which puts conception two weeks after the last period: 280 days from LMP equals 266 days from conception. When cycles are longer or shorter, ovulation shifts, and the rule's accuracy shifts with it, which is why clinicians confirm with first-trimester ultrasound. The trimester cutoffs used here are the standard clinical ones: first trimester weeks 1 to 13, second weeks 14 to 27, third weeks 28 to 40.
Worked Example
Last period: January 1, 2026. Due date = January 1 + 280 days = October 8, 2026. (Check: January 1 is day 1 of the year; day 281 is October 8 in a non-leap year.) First trimester: January 1 to April 1 (weeks 1 to 13). Second trimester: April 2 to July 8 (weeks 14 to 27). Third trimester: July 9 to October 8 (weeks 28 to 40). If instead you know conception happened on January 15: due date = January 15 + 266 days = October 8, the same answer, which confirms the two methods agree.
What the Trimesters Involve
The first trimester is the construction phase: by week 13 the embryo has become a fetus with all major organs formed, though tiny. This is when folic acid matters most and when most miscarriages occur, which is also why many people wait until week 12 or 13 to share the news. The second trimester is often the most comfortable: the anatomy scan around week 20 checks development in detail, and fetal movement (quickening) usually appears between weeks 18 and 22. The third trimester is rapid growth: the brain and lungs mature, weight roughly triples, and prenatal visits become more frequent. Only about 1 in 20 babies arrives exactly on the due date, so think of October 8 as the middle of a normal two-week window on either side. These are estimates, not medical advice: start prenatal care early and follow your clinician's dating.
Frequently Asked Questions
How is a due date calculated?
With Naegele's rule: add 280 days (40 weeks) to the first day of your last menstrual period. The 40 weeks include roughly two weeks before conception, because ovulation is hard to pin down but the last period is a date most people remember. If you know your conception date instead, add 266 days (38 weeks). Ultrasound dating in the first trimester can refine the estimate further.
How accurate is the due date?
It is an estimate, not an appointment: only about 5 percent of babies arrive on their due date. Most are born within two weeks on either side, and first babies often run a few days late. The dating is most accurate when the last period date is certain and cycles are regular; first-trimester ultrasound (crown-rump length) is accurate to about plus or minus 5 days and is what clinicians trust when dates conflict.
What are the trimesters?
The first trimester is weeks 1 through 13: the embryo becomes a fetus and all major organs form. The second trimester is weeks 14 through 27: the anatomy scan happens, movement is felt, and most people feel their best. The third trimester is weeks 28 through 40: rapid growth, lung maturation, and preparation for birth. "Weeks pregnant" is counted from the last period, so at conception you are already considered 2 weeks pregnant.
When should I tell people I am pregnant?
There is no medical rule, only personal comfort. Many people wait until after the first trimester (around week 12 to 13), when miscarriage risk drops sharply and the first ultrasound confirms dating. Others share earlier with close family for support. Do what feels right: the "12-week rule" is tradition, not medicine.
Why does pregnancy last 40 weeks if conception was 38 weeks ago?
Because gestational age is counted from the last menstrual period, not from conception. Ovulation and conception typically happen about 14 days into the cycle, so those two pre-conception weeks are included in the 40-week count. It is a dating convention that keeps everyone (you, your clinician, the ultrasound tech) using the same week numbers.