For most expecting moms, to accurately calculate due date isn’t quite as simple as plotting a nine month forecast. Your baby doc is sure to set you straight after an ultrasound, but when you just gotta know the baby’s birthday now, count down to the big day with our due date calculator.

How to Calculate Due Date

It probably goes without saying, but your due date is based on the day you conceived. You could calculate due date by counting out from your presumed conception date or using a Conception Calculator. However, unless you were really deliberate about tracking your ovulation or were able to make a baby in one shot—and had no extracurricular fun with your man otherwise—knowing which day was the big one isn’t always clear cut.

That’s where a due date calculator, also known as a pregnancy calculator, helps. Based on the first day of your last menstrual period (LMP), the ovulation date is determined and used to forecast when your little one will arrive. While the date may not be 100 percent precise, an egg only has a fertilization window of less than 24 hours, so the due date calculator should give a good estimate of when you’ll be adding a tiny new addition to your home.

There are two exceptions to the pregnancy due date calculator: conception through in vitro fertilization (IVF) or if you’re expecting twins (or other sets of multiples).

Conceiving naturally, or even with the help of fertility drugs, means you can use the due date calculator based on your LMP. However, if you conceive through IVF, it’s best to work with your healthcare provider to determine pregnancy due date. The circumstances surrounding the retrieval and transfer methods will impact the due date calculation by days (if not weeks) when compared to using the LMP.

If you’re having twins and have not already been told the due date by your doctor, you can use the due date calculator based on your LMP just as you would if you weren’t doubling down on babies. However, it’s important to remember that the calculation for solo babies is 40 weeks to full term, whereas many docs consider twins to be full term at 38 weeks. That doesn’t mean the duo won’t hangout for the full 40 (or longer if they really like their space), but just know twins may arrive earlier than the due date noted by the pregnancy calculator.

How to Manually Calculate Due Date

Want to put your superior math skills to good use, or just have a back-pocket parlor trick for run-ins with expecting moms? Skip the due date calculator and put your mental abacus to work with this simple equation:

First day of LMP + 280 days (40 weeks) = Baby Day

Remember that not every month is a strict 30 days, and February likes to throw short-month and leap-year curveballs. Otherwise, you can now use your mind in place of any due date calculator to calculate due date. Though be warned, if you are pregnant, your mommy brain might not be as reliable as usual. Always check your math!

Now that you know what month your baby is about to be born in, you can also get foresight on if the wee one will be a he or she. Use the Chinese Gender Chart to predict baby's gender while you wait for your official gender reveal.