Congratulations, you think
(or know) you might be pregnant! But it's quite confusing when you
try to work out how many weeks pregnant you are.
That's because of the way that doctors and midwives have
traditionally calculated the "weeks pregnant" you are, with 40
weeks being your due date.
Doctors and midwives generally discuss pregnancies in weeks,
rather than months, so they can more accurately assess the progress
and expected milestones of your baby throughout the pregnancy.
The average pregnancy is assumed to be a total of 40 weeks
(that's 280 days) from the first day of your last menstrual period
(often shortened to "LMP.")
Pregnancy is commonly referred to in terms of "gestational age"
rather than "fetal age development." When you think about it
carefully, that means that the average gestation only goes for 38
weeks after fertilisation - and using this system, you are already
two weeks "pregnant"" at the time of ovulation!
To work out how many weeks pregnant you are under the
traditional system (which is still in use in most hospitals in
Australia), just work forward from the first date of your last
menstrual period.
How many weeks pregnant am I - under the fetal age (or
ovulation) system?
Sometimes doctors and midwives talk about the "fetal age" or use
the "ovulation" system of determining how many weeks pregnant you
are, because it is more accurate than just guessing that you might
have ovulated about 14 days after the first day of your last
period.
The fetal age is the actual age of your baby - that is, how many
weeks since the egg and sperm joined together in conception.
Thanks to technologies like ultrasound, as well as
widely-available and far more accurate ovulation testing, fetal age
can be determined much more easily and give you a better estimate
of how many weeks pregnant you are.
If you have a much shorter or much longer menstrual cycle than
the standard 28 days, using the standard gestational age "dates"
won't quite match up with where your baby really should be, so
fetal age can be more accurate.
Trimesters
Pregnancy is usually divided into three "trimesters" of around
twelve weeks each. That's because these represent three quite
different stages of pregnancy in terms of the experiences that the
pregnant woman has, and the development of her baby.
How many weeks will your pregnancy last?
The main reason that most women want to know how many weeks
pregnant they are, is because they want to know what date their
baby will be born.
The traditional method of estimating the due date of a baby,
called Naegele's rule, is to add seven days to the first day of
your last menstrual period, then nine months to the month.
So, if your last period started on 18 November, then your due
date is going to be the 24 August, which works out to around 40
weeks. Naegele's rule assumes the woman has a 28 day menstrual
cycle and ovulated and fell pregnant on day 14; so gestation is 38
weeks from the day of ovulation.
However, things are not that simple; just 3% to 5 % of babies
actually arrive on their due date, much to the frustration of most
expectant parents.
The most common time for babies to arrive is 40 weeks and 3
days; you are statistically far more likely to go overdue than to
go early.
Most professionals argue that babies can be born perfectly
healthily at anything from 37 to 42 weeks after the date of the
last menstrual period.
By Fran Molloy, journalist and mum of four