How To Accurately Calculate Your Baby's Due Date

by Ali Solomon
Originally Published: 
due date
BraunS / iStock

Knowing the magical date when your baby will be born helps you make important decisions: when to take that babymoon, when to start a maternity leave, when to plan a quickie wedding, when to avoid getting stuck in an elevator with a sitcom character. That’s why I’ve invented a secret formula for calculating the exact day of your baby’s birth:

Count 40 weeks from the date of your last period. If you can’t remember it, try to remember the date of the last fight you had with your husband (it’s probably around then).

Subtract three weeks if you are having twins.


Subtract one week if you only look like you’re having twins.

Add two weeks if you are known for being chronically late.

And an extra one if you are 36 weeks pregnant and still haven’t found a doctor who “supports your birth plan.”

Add another week if you like to overthink things. On second thought, maybe subtract a week. No, wait, definitely add one.

Subtract one day if you and your BFF are due on the same date. Of course she’s due the same day as you. Of course you have to be the one to change the date. That’s so typical of her.

Subtract a week and a half if your feet are retaining more water than a sea sponge and you would like nothing more than to remove this succubus immediately.

Add a day if the crib you ordered from IKEA hasn’t arrived yet.

Add six more days if you have no idea how to assemble it.

Subtract a week if you’ve already had a baby.

Add a week if the reality of having two children just hit you, and you’d like more time, please more time, just one more week before your life succumbs to chaos.

Subtract three days if your husband asked that your impending labor not “ruin St. Patrick’s Day” for him.

Use the online lunar birth date predictor your friend linked you to on Facebook. While you’re at it, calculate your baby’s gender.

A boy? Impossible—you’re carrying in the front, dammit! Take the test again, this time factoring in a waning crescent moon.

Add a week if you won tickets to a Bon Jovi concert on your actual due date, and you really, really want to attend.

Add another week if your sister requested that you not have your baby on her birthday.

Or her wedding day.

Take the number of days you’ve been pregnant, multiply it by the number of months you plan on breastfeeding, and divide that by the amount of paid maternity leave your job provides.

Congratulations! You have successfully calculated your exact due date. Your baby will arrive sometime within six weeks before or after that date. Happy planning!


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