Due Date Calculator: When Is Your Baby Due?
Our due date calculator uses the following methods to calculate your due date.
Due Date Based on Your 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.
Due Date Based on 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.
Due Date Based on IVF Transfer
If your pregnancy is the 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. Due Date by 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.
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