Most pregnancies last around 38 weeks from conception. Typically women ovulate about two weeks after their menstrual cycle starts, so the best way to estimate your due date is to count 40 weeks, or 280 days, from the first day of your last menstrual period.
Another way to do it is to subtract three months from the first day of your last period and add seven days. So if your last period started on April 11, you’d count back three months to January 11 and then add seven days, which means your due date would be January 18.
This is how your doctor will estimate your due date — and it’s a pretty solid target. But remember: It’s just as normal to deliver a week or two before or after.
You have several options for determining your conception date:
- An ultrasound is the most accurate way to see how far along your pregnancy is. Ultrasounds provide a detailed look at your baby’s development and will give you a good estimate of how far along your pregnancy is.
- An online pregnancy calculator can give an estimate of when you conceived based on when your last period was. This method is not as accurate as a confirmation ultrasound.
We offer free ultrasounds at our pregnancy center. Our medical professionals are trained in limited obstetrical ultrasound and provide medical services under the direction and supervision of a licensed physician.
How to Know When You Conceived
In general, women can only conceive when they are ovulating – that is, when their ovaries release a new egg into their fallopian tubes for fertilization. Ovulation usually occurs about two weeks after your menstrual period and lasts for two or three days.
However, not all women have the same cycles of menstruation and ovulation. Some women’s menstrual cycles are irregular. Others have ovulation periods closer to or further from their menstrual period. Some women simply aren’t sure when their last period was. In other words, calculating conception based just on your body cycles can be difficult.
You can also simply try to determine the last time you had sexual intercourse, but this method isn’t totally accurate either. Conception is a process and takes time, and a man’s sperm can actually stay alive inside your body for up to a week after sex. All of these factors can make it hard to know when exactly you conceived.
This is why a pregnancy confirmation ultrasound is the best way to know how far along your pregnancy is and to determine when you conceived. Pregnancy ultrasounds look directly at the development of your growing baby to determine its age and when you likely conceived. This method is much more accurate than date-based calculation.
Are you expecting? Use this pregnancy due date calculator to find out when your baby is due. Your health care provider will confirm the baby's gestational age with a first trimester ultrasound.
How is my due date calculated?
Conception Date: If you have been using an ovulation predictor kit or tracking your ovulation you may be able calculate your due date based on the precise date of conception. In order to do this simply count 266 days, or 38 weeks, from the conception date to find your estimated due date.
Last Period: This is the most common way to calculate your estimated due date. To calculate, count 40 weeks, or 280 days, from the first day of your last menstrual period.
IVF: If you conceived through IVF, you can calculate your estimated due date using your IVF transfer date. If you had a day-three transfer, count 263 days from the transfer to your estimated due date, and for a day-five transfer, count 261 days.
How to Calculate Conception Date
According to Naegele's Rule, the average human gestation period is 266 days. Therefore, to figure out the conception date from a birth date we need to subtract 266 days from the due date (or birth date).
But since it's difficult to subtract days from a calendar date without a date calculator, the easiest way to determine conception date by birth date manually is to count backward on a calendar.
Here is a conception calendar calculator that illustrates counting from 1 to 266 days backward from the due date.
Due DateConception Date1st Day of Last Period (LMP)Possible Conception Date | ||
How to Plan Baby Birth Date
If you select the desired due date in the Reverse Due Date Calculator on this page, the calculator will tell you when you will need to get pregnant to increase the odds of delivering on the desired date.
If you would rather know the month ranges, here is an at-a-glance chart showing target conception ranges for delivery months.
January baby | Apr 10 - May 10 (Apr 24) |
February baby | May 11 - Jun 7 (May 24) |
March baby | Jun 8 - Jul 8 (Jun 21) |
April baby | Jul 9 - Aug 8 (Jul 22) |
May baby | Aug 9 - Sep 7 (Aug 21) |
June baby | Sep 8 - Oct 7 (Sep 21) |
July baby | Oct 8 - Nov 6 (Oct 21) |
August baby | Nov 7 - Dec 7 (Nov 20) |
September baby | Dec 8 - Jan 6 (Dec 21) |
October baby | Jan 7 - Feb 6 (Jan 20) |
November baby | Feb 7 - Mar 9 (Feb 21) |
December baby | Mar 10 - Apr 9 (Mar 23) |
Of course, there is no guarantee whether or not your baby will be born on its due date. So the best chance for having the baby in the desired month is to plan conception in the middle of the ranges listed above.