Pregnancy

Due date calculator

Enter the first day of your last period and your usual cycle length to estimate your due date, how far along you are today and when each trimester begins. The calculator uses the same last-period method obstetricians use, and an early ultrasound takes over whenever the two dates disagree.

First day of your last periodThe date your last period started.Average cycle lengthdaysFrom the first day of one period to the first day of the next. The calculator accepts 21 to 35 days; leave 28 if you are not sure.

Enter your values above to see your result and what it means.

How your due date is calculated

By convention, the estimated due date is 280 days (40 weeks) after the first day of your last menstrual period. This is Naegele's rule, and ACOG notes that it assumes a regular 28-day cycle with ovulation about 14 days after the period starts. When your cycle is longer or shorter, ovulation shifts by the same number of days, so this calculator moves the due date to match: a 32-day cycle adds 4 days, a 25-day cycle takes off 3. Weeks of pregnancy are counted on the same calendar, which is why you are already "2 weeks pregnant" around the time you conceive. The table shows due dates for a 28-day cycle when the last period started on the 1st of each month.

Due date for a 28-day cycle by first day of the last period (280 days later). When the 280 days include February 29, the due date falls one day earlier.
Last period started	Estimated due date
January 1	October 8
February 1	November 8
March 1	December 6
April 1	January 6
May 1	February 5
June 1	March 8
July 1	April 7
August 1	May 8
September 1	June 8
October 1	July 8
November 1	August 8
December 1	September 7

Pregnancy weeks and trimesters

ACOG divides pregnancy into three trimesters by completed weeks and days: the first trimester runs up to 13 weeks 6 days, the second from 14 weeks 0 days to 27 weeks 6 days, and the third from 28 weeks 0 days. For births from 37 weeks, ACOG and the Society for Maternal-Fetal Medicine use four labels in place of the single word "term". Your current week of pregnancy is one more than the completed weeks: at 12 weeks and 3 days you are in your 13th week.

Trimesters (ACOG Committee Opinion 700) and term categories (ACOG Committee Opinion 579).
Stage	Weeks of pregnancy
First trimester	Up to 13 weeks 6 days
Second trimester	14 weeks 0 days to 27 weeks 6 days
Third trimester	From 28 weeks 0 days
Early term	37 weeks 0 days to 38 weeks 6 days
Full term	39 weeks 0 days to 40 weeks 6 days
Late term	41 weeks 0 days to 41 weeks 6 days
Postterm	42 weeks 0 days and beyond

When an ultrasound changes your due date

Dating from your last period has known weak spots: ACOG cites reports that only about half of women recall the date accurately, and cycles and ovulation vary. A first-trimester ultrasound that measures the embryo from crown to rump is accurate to within about 5 to 7 days, so ACOG uses it to confirm or replace the period-based date. The table lists the differences at which ACOG recommends switching to the ultrasound date. If your pregnancy came from IVF, the due date is set from the embryo transfer date instead.

Guidelines for redating based on ultrasound, ACOG Committee Opinion 700, Table 1.
Pregnancy stage at the ultrasound	Change the due date if the scan differs by
Before 9 weeks 0 days	More than 5 days
9 weeks 0 days to 13 weeks 6 days	More than 7 days
14 weeks 0 days to 15 weeks 6 days	More than 7 days
16 weeks 0 days to 21 weeks 6 days	More than 10 days
22 weeks 0 days to 27 weeks 6 days	More than 14 days
28 weeks 0 days and beyond	More than 21 days

How accurate is a due date?

It marks the middle of a range. In an Australian study of 18,708 women who went into labor on their own, only 5% gave birth on the estimated date, whichever dating method was used, and about 66% gave birth within 7 days of it. Natural variation is wide even when the date of conception is known exactly: in a study that pinpointed ovulation with daily hormone tests, the median time from ovulation to birth was 268 days (38 weeks 2 days), and births spanned 37 days even after preterm births were excluded. Think of the due date as the center of a window of several weeks.

Frequently asked questions

How is my due date calculated?

Add 280 days (40 weeks) to the first day of your last period. That assumes a 28-day cycle; if your cycle is longer or shorter, add or subtract the difference. A last period that started on January 1 with a 28-day cycle gives a due date of October 8. An early ultrasound can then confirm or change the date.

How far along am I?

Count the days since the first day of your last period and divide by 7: the whole number is your completed weeks and the remainder is extra days. Eighty-seven days is 12 weeks and 3 days, which means you are in your 13th week. The calculator does this for you and adjusts it for your cycle length.

Can my due date change?

Yes, usually once, early on. If your first ultrasound differs from your period-based date by more than ACOG's threshold for that stage (more than 5 days before 9 weeks, more than 7 days up to 15 weeks 6 days), your care team switches to the ultrasound date. After that, ACOG advises changing it only in rare circumstances.

What are the chances of giving birth on my due date?

Small. In a study of 18,708 births after spontaneous labor, 5% happened on the estimated due date and about two thirds within a week either side of it.

When did I conceive?

Probably around the time you ovulated, which in a 28-day cycle is about 14 days after the first day of your last period, or 266 days before your due date. The calculator shows this estimated conception date, adjusted for your cycle length. Ovulation timing varies, so treat it as an estimate within several days.

Should I use this due date or the one from my ultrasound?

Use the one from your ultrasound and your care team. ACOG names a first-trimester ultrasound as the most accurate way to establish the due date, and the date your midwife or doctor records is the one used for your care.