Pregnancy Calculator

Pregnancy calculator at a glance#

A pregnancy calculator estimates your due date and how many weeks pregnant you are from the first day of your last menstrual period (LMP). The standard method, Naegele’s rule, adds 280 days (40 weeks) to the LMP, which is the same as counting back 3 months from the LMP and adding 7 days.

Worked example: if your LMP started on January 1, add 280 days to get an estimated due date of about October 8. By the subtraction form, January 1 minus 3 months is October 1, plus 7 days is October 8, the same result.

Gestational age is counted from the first day of your LMP, not from conception, so at the moment of conception you are already counted as about 2 weeks pregnant. To find how many weeks pregnant you are, take the number of days since your LMP and divide by 7: 70 days since the LMP is exactly 10 weeks. The pregnancy spans about 40 weeks, grouped into three trimesters of roughly 13 to 14 weeks each.

Enter your last period date, conception date, ultrasound date or IVF transfer date in the calculator above for your estimated due date and current week. This is an estimate based on a regular 28-day cycle, so an early ultrasound, a longer or shorter cycle, or individual variation can shift it. Fewer than 1 in 25 babies arrive on the exact due date, and only your healthcare provider can confirm dating, so use this as a planning guide rather than medical advice.

How the due date is calculated#

The standard method adds 280 days (40 weeks) to the first day of your last menstrual period (LMP), which assumes a regular 28-day cycle. The same date drops out of other inputs the calculator accepts: from a known conception or ovulation date it counts forward 266 days, and an early ultrasound or IVF transfer date dates the pregnancy directly. Maternal age, cycle length, and individual variation all affect the actual length, and fewer than 1 in 25 babies arrive on the exact estimated date.

Gestational age and trimesters#

Gestational age is counted from the first day of your LMP, not from conception, so at the moment of conception you are already counted as about 2 weeks pregnant. To find how many weeks you are, divide the days since your LMP by 7. A full-term pregnancy runs 37 to 42 weeks and is split into three trimesters: weeks 1 to 13, weeks 14 to 27, and week 28 to birth, each with its own developmental milestones.

Confirming and adjusting your dates#

A missed period is the usual prompt to take a pregnancy test; home urine tests and blood tests detect the hormone hCG and can pick up a pregnancy about a week or two after a missed period. An early ultrasound, usually around 7 to 8 weeks, often dates the pregnancy more precisely and can shift the due date. If you do not know your LMP, a conception date or an early scan can be used instead.

Pregnancy calculator FAQ#

How does a pregnancy due date calculator work?#

It adds 280 days (40 weeks) to the first day of your last menstrual period, or 266 days to the conception date. The result is an estimate based on a regular 28-day cycle, so a longer or shorter cycle moves it.

How accurate is the estimated due date?#

It is usually within a week or two of the actual birth, but only about 4% of babies arrive on the exact date. An early ultrasound gives the most reliable dating.

How do I find out how many weeks pregnant I am?#

Count the days since the first day of your last period and divide by 7. For example, 70 days since your LMP is exactly 10 weeks. Weeks are counted from the LMP, not from conception.

Can the due date change after an ultrasound?#

Yes. A first-trimester ultrasound measures the baby directly and can adjust the due date, especially if your cycle is irregular or you are unsure of your LMP.

What if I do not remember my last period?#

Use your conception or ovulation date if you know it, or have an early ultrasound. A scan in the first trimester can estimate gestational age and set a due date without the LMP.