Pregnancy Due Date Calculator

Our due date calculator uses the following methods to calculate your due date.

Last Period

This method calculates your due date based on the first day of your last period, assuming a typical pregnancy, on average, lasts 40 weeks (or 280 days) from the first day of your last period. To calculate your due date using this method, you also need to input the average length of your menstrual cycle.

If your menstrual cycle is longer than the average cycle of 28 days, your due date is pushed forward and the amount of time you’re pregnant is reduced. Meanwhile, if your cycle is shorter than the average 28 days, your due date is pushed back.

Conception

This method calculates your due date based on the date of conception by assuming a gestation period of 38 weeks (or 266 days) from the date of conception.

Now, Kelly Culwell, M.D., an obstetrician-gynecologist and Forbes Health Advisory Board member, notes that the date of conception isn’t always a reliable method for determining your estimated due date. It’s “often incorrectly estimated due to [the] date of intercourse, which can be several days off from [the] actual date of conception based on the sperm’s ability to survive for five days in the reproductive tract,” she says.

IVF Transfer

If your pregnancy is a result of a successful in-vitro fertilization (IVF) procedure—in which eggs are extracted and fertilized with sperm, with the resulting embryo later transferred to the uterus—you can calculate your estimated due date based on the timeline of the procedure.

• IVF day 3 transfer: If your embryo was transferred to your uterus after three days of embryo growth, this method calculates your estimated due date based on 263 days from your date of transfer.
• IVF day 5 transfer: If your embryo was transferred to your uterus after five days of embryo growth, this method calculates your estimated due date based on 261 days from your date of transfer.

Ultrasound

This method of calculation takes into account the weeks and/or days in which you had an ultrasound. It takes the date of your ultrasound, subtracts the number of weeks at which your ultrasound is done and then adds 280.

Not all the methods described above are the same in terms of accuracy, notes Dr. Culwell. “One of the biggest misconceptions that I see is pregnant people thinking that their due date changes if the dating is slightly different on later ultrasounds, or not believing a due date should change if the first trimester ultrasound is different from what is predicted based on their last menstrual period.”

Instead, she says the most accurate method of calculation (aside from IVF transfer dates, which are the most accurate) is an early first trimester ultrasound and certain date of last menstrual period, followed by the date of conception and lastly, a second or third trimester ultrasound.

It can be difficult to detect pregnancy without a test, as many of the early signs and symptoms mirror those of your period. The most common early signs of pregnancy include:

• A missed period
• Frequent urination
• Fatigue
• Nausea
• Sore and enlarging breasts

Less common early symptoms of pregnancy include cramping, spotting (or light vaginal bleeding), food cravings or aversions, headaches and dizziness, fluctuating moods and a metallic-like taste in your mouth.

Typically, an ultrasound is used to confirm your due date (based on your estimated due date calculated from your last menstrual period), and “once a due date has been selected, it does not change no matter how many additional ultrasound exams you may have during your pregnancy,” states ACOG.

Still, ACOG also notes that “subsequent changes to the EDD (estimated due date) should be reserved for rare circumstances, discussed with the patient and documented clearly in the medical record. A pregnancy without an ultrasound examination that confirms or revises the EDD before 22 0/7 weeks of gestational age should be considered suboptimally dated.”

Though due dates can be helpful to give an estimated time of arrival for expecting parents, they are rarely accurate to the day, according to ACOG.

Factors that can contribute to your baby being overdue—or a postterm pregnancy—are often unknown, but can include:

• If this is your first baby
• If you’re carrying a male fetus
• If you’ve had a previous postterm pregnancy
• If you have obesity

Factors that can lead to preterm birth of your baby (birth before 37 weeks) include:

• Your medical history, such as having a previous preterm baby
• Pregnancy complications, including carrying more than one fetus
• Lifestyle factors, including if you smoke
• If you’re under the age of 17 or over the age of 35

No estimated due date is ever 100% accurate, and even ultrasound scans aren’t entirely accurate. However, the ACOG notes that the most accurate method of confirming gestational age is an ultrasound measurement of the fetus or embryo in the first trimester, citing an accuracy of +5-7 days.