|Dating a Pregnancy|
A pregnancy is usually referred to in weeks, starting
from the first day of the last menstrual period (LMP). However, as
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- (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 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 .
The information in this page is presented in summarised form and has been taken
from the following source(s):
Hygeia Foundation :
Introduction to Human Physiology , 2nd Edition (1981), M. Griffins, Macmillan Publishing
Co. Inc, New York
Child Development , 6th Edition (1994), J. W. Santrock, Wm. C. Brown
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