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Dating a Pregnancy

A pregnancy is usually referred to in weeks, starting from the first day of the last menstrual period (LMP). However, as ovulation generally occurs about 2 weeks into the menstrual cycle, the developing foetus is thus 2 weeks younger than the number of weeks a woman is referred to as being pregnant. This means a woman who is 6 weeks pregnant is carrying a 4 week old foetus.

The average pregnancy is about 266 days post- conception (38 weeks) or 280 days (40 weeks) after the first day of the last period. Most women give birth within 2 weeks either side of the expected date.

However, sometimes the date of conception can't be determined because a woman's menstrual periods are irregular or she isn't sure about the length of time between them. In cases like these, an ultrasound examination may be undertaken early in pregnancy, which can help determine the duration of pregnancy. Later, but before 32 weeks (ideally between 18 and 22 weeks), a series of ultrasound examinations to measure the diameter of the foetus' head can help confirm the duration of the pregnancy. After 32 weeks, determining duration by ultrasound examination may be 3 weeks off in either direction.

Pregnancy is typically divided into three 3-month periods, or trimesters .
The first trimester is from week 1 to 12, the second between weeks 13 and 24, and the third from week 25 until birth .

See also preterm and postterm labour .

The information in this page is presented in summarised form and has been taken from the following source(s):
1. Hygeia Foundation :
2. Introduction to Human Physiology , 2nd Edition (1981), M. Griffins, Macmillan Publishing Co. Inc, New York
3. Child Development , 6th Edition (1994), J. W. Santrock, Wm. C. Brown Communications, Inc.

Other HON resources 
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Dating a Pregnancy
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Pregnancy Trimesters

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Pregnancy Trimesters


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Contact Last modified: Jun 25 2002