Pregnancy & Baby Index: Pregnancy - Week by Week: Confused about "age since conception," LMP and due dates?

Confused about "age since conception," LMP and due dates?

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We know it can be a little mystifying -- you practically need a calculator to figure out your due date, conception date and LMP date. One common question is, "How is my due date calculated" -- and another biggie: "What's the difference between gestational age and menstrual age." Read on for a little insight.

Week one -- when we're talking about the typical 40 weeks of pregnancy -- starts on the first day of your period. Yes, it's true that you're not actually pregnant yet, but most caregivers count everything from this day, anyhow. The number of weeks passed, based on the 40 week term from your last menstrual period (also known as your "LMP" date), is called "menstrual age."

Two weeks later, when -- and if -- one of your eggs is fertilized, "gestational age" begins to mark time from baby's conception. This term lasts about 38 weeks, which is the amount of time your baby is actually developing.

"Naegle's Rule" is a simple formula to calculate your estimated due date, based upon the fact that the average pregnancy lasts 280 days from the last period (or 266 days from conception). To apply the rule and work out when your baby is due, add 7 to the date (day number) of your LMP (last menstrual period), subtract three months, then add a year. For example:

If you had a LMP date of June 10th 2007:

1. Add 7 days to the 10th = 17th
2. Take off 3 months from June (6th month) = March (3rd month)
3. Add one year: 2007 + 1 = 2008

Your due date would be March 17, 2008! (And your date of conception would be June 24, if you typically have a 28-day cycle.)

Looking for something involving a little less math? Use our due date calculator for instant results. Or, to find your conception date, click here!

But don't forget...
Now that you know all of this information, it's also important to be aware of one more thing: A due date is an estimate only, as every baby -- just like every human -- is different. (Think of it this way: Did you have a growth spurt or start your period at exactly the same time your friends did?)

In reality, only about five percent of babies are born on what is technically considered their due date. Most babies are born anywhere between two weeks before and two weeks after that date -- and they're all term, healthy and perfect in the way nature intended.