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Calculating Your Due Date

Mari pregnant with twins
The first thing I wanted to know when I realized I was pregnant was WHEN??!! When is the baby coming? I wanted to know if I'd be planning a Christmas baby or a hot summer surprise. My mind sped into the future to a day on a calendar - and focused on that day for months. I dreamt about it, prayed about it, feared it and waited . . . Yes, "when" was the most important question in my mind at that moment.

My husband is a normally rational man. He thinks logically and clearly quite like Mr. Spock. I appreciate this quality in him. However, all logic aside, when it came to hearing the big news, his reaction was usually something like this . . .

Are you sure?

     followed by

        I'm stunned

           followed by another

               are you sure?

                 and then


                       followed by another

                          I'm stunned!

I never really understood how he could be so shocked because he was always very aware of when we were trying to conceive, aware of when we failed, and aware of when we were in that waiting period wondering are we? He was always a part of the decision-making process of should we try again so the I'm stunned comment always made me wonder what happened to produce this particular case of amnesia. Given his state of shock, I'm amazed he didn't ask me how did this happen?!

Perhaps it was my uncreative way of telling him . . . Greg, I'm pregnant. Somehow, I think those three words sounded like Greek to him. But isn't blunt the way you talk to Mr. Spock? A rational man should realize that unprotected sex can and does result in conception, which if everything goes as planned, produces a baby some 38 weeks later. Once the idea soaked in and he realized that he was that man in the bedroom with me, he was as excited as I was. This probably took all of five minutes, but it was a LOOONNNGGG five minutes for me. Hello?? Hello?? Mr. Spock, where are you?

A little foresight into spilling the beans could have gone a long way in preventing the amnesia. StorkNet has some great ideas from moms on how they creatively told their families. I wish I'd thought of some of these ideas!

So you've told your partner that you're going to become parents, and you work your way through the shock, surprise, stunned comments and amnesia to the big when question. In calculating your due date, there are a few things to keep in mind. First, your due date is nothing more than a guess. Out of 563 expectant couples I tracked through StorkNet, only 22 delivered on their actual due date. Babies exercise their independence at an early age and generally come when they want to.

Your doctor may refer to your due date as the "EDC" which stands for estimated date of confinement (a very OLD term!) or "EDD," estimated due date. This is where due date calculation gets messy. There are so many terms thrown around . . . gestational age, calendar age, fetal age, menstrual age, lunar month, 9 months, 10 months, 40 weeks, LMP. It boggles the mind how they make something so difficult over a little word like when. Here's the scoop . . . most care providers will calculate your due date based on the first day of your last menstrual period (LMP). Your baby was conceived around 14 days after the first day of your LMP. This means that when you actually conceived, the timer had already started ticking, and you were about two weeks pregnant the day you conceived. Full term is 40 weeks - that's 40 weeks after the first day of your last menstrual period or 280 days after LMP or 38 weeks since conception or 266 days since conception. All of those time frames do add up to the same due date, believe it or not! Forty weeks is 10 lunar months, a lunar month being four 7 day weeks. Of course, all of this calculation is based on the fact that women are SUPPOSED to have 28 day menstrual cycles. If you're like most women and have a different cycle or an irregular cycle, then your EDD is really a guess at best.

Confused? Don't feel bad - that's why doctors use charts to calculate due dates, and when there's the possibility that you didn't conceive right at 14 days LMP, an early ultrasound can be the most accurate due date predictor. But, if you want to figure a ballpark date now before you visit your care provider, grab a calendar and count 40 weeks from the first day of your last menstrual period or add seven days to the first day LMP and then count back three months. If you're wailing with frustration, don't despair. I've made a handy dandy due date chart and glossary for you. I remember early pregnancy brain fog very well!

P.S. As my husband read this article over my shoulder, he pointed out that I have a slight case of amnesia too. He reminded me that he did, indeed, ask me HOW DID THIS HAPPEN? How quickly we forget!


EDC - Estimated Date of Confinement

EDD - Estimated Due Date

Full term - 37 to 42 weeks gestational age

LMP - Last Menstrual Period

Post-term pregnancy - aka overdue and I want this baby out - over 42 weeks gestational age

Premature - a baby born before full term or 37 weeks gestation

Trimester - pregnancy is broken up into 3 phases which are usually about 3 months each

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