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Estimating Your Due Date

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Pregnancy related image Photo: Getty Images

If you’ve just had a positive pregnancy test you may be wondering when your baby is due. Doctors class pregnancy as being 280 days in length, or 40 weeks which is nine calendar months (because there is just a little bit over four weeks in each month).

Since no one knows exactly what day you conceived, your start date for pregnancy will be recorded as the first day of your last menstrual period (LMP). This is because nearly all women remember this date so doctors use it to estimate the number of weeks pregnant.

In actual fact, you won’t be pregnant on the first day of your last menstrual period and it’s fairly unlikely that it would occur until two weeks after that date, at the start of ovulation.

The baby’s age is referred to as its ‘gestational age’ when calculated by this method.

For instance, if your last menstrual period was 30th May 2011 then your due date will be 6th March 2012.

Your doctor can work out your expected date of delivery (EDD) for you or there are many pregnancy calculators online that will estimate your due date at the click of a button. Alternatively if you want to work it out yourself, you can:

Find the date for your LMP, add one week, subtract three months and then add one year.

For instance, for a LMP of 30th May it works like this:

30th May + one week = 6th June
6th June – three months = 6th March
6th March 2011 + one year = 6th March 2012

The Lunar Pregnancy Calculator

That doesn’t mean the pregnancy symptoms have driven you mad. It is another method of calculating how pregnant you are, based upon the conception date as the start of pregnancy, which is actually more accurate. The method, however, is not used in the United States. The lunar method is mainly used in Far Eastern countries.

You calculate this date by starting with the date of conception and adding 266 days to give you the EDD. Your baby’s age is classed as "fetal age" when using this method of calculation and will be born around 38 weeks of age.

Both methods are only a guide as only around 5 percent of babies are actually born on their due date.

Normal full term pregnancy usually lasts anywhere from 37-42 weeks gestation, using the LMP method, so don’t worry if your baby hasn’t made an appearance on his due date. He soon will so you should rest while you still can!


The American Congress of Obstetricians and Gynaecologists, Your Due Date. Web. 29 August 2011. http://www.acog.org/publications/patient_education/bp069.cfm

Baby 2 See, Pregnancy Length. Web. 29 August 2011. http://www.baby2see.com/pregnancylength.html

Joanna is a freelance health writer for The Mother magazine and Suite 101 with a column on infertility, http://infertility.suite101.com/. She is author of the book, 'Breast Milk: A Natural Immunisation,' and co-author of an educational resource on disabled parenting. She is a mother of five who practised drug-free home birth, delayed cord clamping, full term breast feeding, co-sleeping, home schooling and flexi schooling and is an advocate of raising children on organic food.

Reviewed August 30, 2011
by Michele Blacksberg R.N.
Edited by Jody Smith

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We value and respect our HERWriters' experiences, but everyone is different. Many of our writers are speaking from personal experience, and what's worked for them may not work for you. Their articles are not a substitute for medical advice, although we hope you can gain knowledge from their insight.



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