Your estimated due date comes from Naegele’s rule, a standardized calculation in which 280 days are added to the first date of your last menstrual period. It’s not at all specific to you or your body, and there’s a lot of variability. However, it gives you and your obstetrician an idea of generally what to expect regarding the timeline of your pregnancy.
To calculate an estimated due date, follow these steps:
- Determine your date of conception. This date is between 12 and 24 hours after the date of your last ovulation. Unless you’ve undergone in vitro fertilization, the calculation is likely to be imprecise.
- From this date, add 280 days (or nine months and seven days). That’s your estimated due date.
- If you’ve conceived through in vitro fertilization (factoring in the number of days your embryo has grown prior to your embryo transfer) or know the precise date of conception, instead of 280 days, add 266 days to that date. That’s your estimated due date.
If you haven’t been tracking your ovulation or aren’t sure of the date of conception, there’s another method to calculate your due date based on your last menstrual period.
To calculate using your last menstrual period, follow these steps:
- Determine the first day of your last menstrual period (LMP).
- Count back three months from that date (and be mindful of which months have 28 or 31 days).
- Add 372 days (one year and seven days) to that date. That’s your estimated due date.
Common questions about due dates
- How do I figure out my conception date?
- Your egg is capable of supporting fertilization by sperm for approximately 12 to 24 hours after you ovulate. If you’re pregnant, it’s fairly safe to assume that you became pregnant within a day of your last ovulation. If you’re unsure of the dates, schedule an appointment to see an obstetrician.
- What if I don’t know the date of my last menstrual period?
- If you can’t recall the day, the next step is an ultrasound, which can identify a pregnancy as early as four weeks after your LMP. However, an embryo may not be visible until after five weeks.
- What if I have longer cycles?
- If you have a longer but predictable cycle, you can factor that into one of the calculation methods above. You may also opt to use a due date calculator.
- What if I have irregular periods?
- You can try to do the math based on Naegele’s rule and what you know about your body, but your best bet is to schedule an appointment with your medical provider and get an estimate with an ultrasound.
- Can my due date change?
- Yes. The dates you provide, the number of embryos you’re gestating and your genetic heritage can be a factor in determining the estimated due date.
- Why do due dates change?
- A due date change is unlikely if you have regular 28-day menstrual cycles but more common if you have an irregular cycle. Your due date will be affected by probably no more than a week or two, but other factors may elongate your pregnancy, including your birth history and your family’s birth history.
- Will I give birth on my due date?
- The odds of giving birth on your due date are very low. No pregnancy is likely to result in birth on an on-time due date. If your baby arrives on a date you and your obstetrician expect, it may be the result of improved technology, or your baby’s birth date may be the result of a planned induction or C-section.
- How do twins or multiples affect due dates?
- If you’re having twins or a higher-order gestation, your largest embryo will determine the estimated due date.
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