What Due Date

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 this date.

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

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.

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.