At the end of your first prenatal visit you will usually
be informed of your EDD or estimated date of delivery.
(some books define EDD as expected date of delivery).
EDD should be regarded only as an estimate. It is
really the midpoint of a 4 week period with the baby
either arriving two weeks before or two weeks after
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
• 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
• 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
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
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.
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.
EDD calculation from
First Fetal Movements
• Another method is to count 18-20 weeks from
the time the first fetal movements or the first
flutters of life are felt. (at about 20-22 weeks
with a first baby and 16-18 with subsequent babies)
• This method is not very accurate with the
main reason being the mother's perception of fetal
movement. Many are not sure and tend to confuse
these movements for something else.
Factors affecting Delivery Date
There are several factors which may influence when
within the 280 day cycle the delivery may happen.
• Menstrual cycle:
Women with consistent and regular menstrual cycles
more or less have their baby on day 280 than the
woman who menstruates irregularly. A short menstrual
interval e.g. 25 days is frequently associated with
delivery a few days earlier. That is to say counting
from the first day of bleeding in one cycle to the
first day of the next cycle is linked to early delivery.
Conversely a lengthy menstrual interval is associated
with a delivery date later than the EDD.
• Birth weight:
A study showed that heavier babies i.e. babies weighing
more than 10 lbs were delivered about a week or
2 past day 280.
• Multiple births:
On the average twins were delivered about 4 weeks
early; triplets arrived about 7 weeks earlier and
quadruplet pregnancies being even briefer.