Due date Calculation
• Since most months are longer than
4 weeks, it is not that simple to make the
calculation.
• Fewer than 5% babies actually arrive
on the due date with most being born before
or after the due date.
• The most common length of pregnancy
is 280 days after the start of the last
period. This adds up to 40 weeks or 9 months
and 1 week
 •
Counting from the day of fertilization,
pregnancy duration averages at 266 days
or 38 weeks or 1 week less than 9 months.
• Naegele's Rule
has this formula which is normally used
by doctors to arrive at the EDD. Add 7 days
to the first day of the last normal period
and count back 3 months. For example March
15 was the first day of your last menstrual
period or LMP. 15 +7 = 22. Count back three
months - February, January, December. Your
expected due date is December 22.
• Wheel Calculators are used by some
doctors to figure out the EDD. They are
compact, low tech devices which calculate
when the baby is due on the basis of the
woman's LMP. The wheel has the added feature
of tracking fetal development in length
and weight for each week of pregnancy and
also takes into account the varying number
of days in each month.
• EDD calculation using the Naegele
Rule or the wheel calculator only applies
in situations where the woman is sure of
the start of her LMP and whose periods are
fairly regular
• 280 days is the average length of
pregnancy. A good number of pregnancies
end before day 280 and after it; a relatively
small number of pregnancies end on the exact
date
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EDD
calculation from Ultrasound
Using ultrasound to determine the
EDD is useful in the following situations:
• When a woman isn't sure of the date
of her last menstrual period
• When a woman has very irregular
periods
• When the size of her uterus doesn't
match the size expected for her last menstrual
period
• When a woman registers late for
her prenatal care
Ultrasound determines the number
of weeks of the fetus at the time of the
test.
• A transvaginal ultrasound can estimate
the age of the embryo at 6 weeks of gestation.
• From 7 weeks onwards, the sonographer
can measure the crown-rump length of the
embryo and judge the gestational age. From
weeks 10-26 the crown rump length or CRL
can be used to determine fetal age. The
most accurate time to use ultrasound as
a dating device to scan the gestational
age and delivery date is either between
7 and 10 weeks of pregnancy or 12 and 18
weeks.
EDD
calculation from Uterine Size
• Uterus size can be assessed with
an error of about 2 weeks on either side.
• The uterus size of a woman expecting
her 2nd or 3rd child may be larger than
the uterus of a first time mother.
• Bearing in mind that these comparisons
are rough estimates at best, a uterus at
6 weeks can be compared to the size of a
lemon; at 10 weeks it can be compared to
a grapefruit and by 12 weeks the uterus
size can be compared to a cantaloupe.
• After 20 weeks doctors use measuring
tape to measure the uterus. Between weeks
20 and 36 the measurement is from the pubic
bone to the top of the uterus or fundus
which equals to the number of gestation
weeks. |
