As soon as pregnancy is confirmed, most women will want to ascertain the expected date of delivery. Calculating the pregnancy due date is usually based on the last menstrual period (LMP), but may be confirmed or corroborated by other means. The estimated date of delivery (EDD) or the estimated date of confinement (EDC) is a specific date arrived at by certain calculations. It is by no means the most accurate of calculations, in the sense that very rarely is a baby born on exactly that date. A variation of two weeks either way is considered quite normal and is more often the rule than the exception.
Methods of Calculating Pregnancy Due Date
The pregnancy due date can be determined by any of the following methods, some that can be done at home by the expectant mother. However, a doctor will determine the final due date taking more than one factor into consideration, such as confirming the due date obtained by calculation from the LMP with the results of an ultrasound examination.
Date of Last Menstrual Period
Normally, a pregnancy will last for 280 days or 40 weeks, which is equivalent to 9 calendar months or 10 lunar months, calculated from the first day of the last menstrual period (LMP). EDC can be calculated accordingly, by adding 280 days to the first day of the LMP.
In a woman with a 28 day cycle, calculation may also be done as 266 days or 38 weeks from the last ovulation (presuming that ovulation and conception occurred 14 days after the first day of the LMP). By adding 14 days to the first day of the LMP and then further adding 266 days to it, the expected delivery date can be calculated.
This method may not always be accurate because cycle lengths vary greatly in women.
Date of Conception
If a woman is sure about her date of conception, the EDC can be calculated by adding 266 days to it.
Date of Ovulation
Women who have been trying to conceive often keep a record of their basal body temperature (BBT) and luteinizing hormone (LH) as a means of determining their date of ovulation. If the exact date of ovulation is known, confirmed by a rise in BBT or an LH surge, you just need to add 266 days to it in order to calculate your pregnancy due date.
Pregnancy Calendars or Obstetric Wheels
These may be the easiest methods of determining the expected date of delivery. Speak to your pharmacist or gynecologist about these tools.
Either the simple old-fashioned ones or the electronic pregnancy calculators can easily calculate the EDC when you enter the first day of the LMP. Computer software or web-based online calculators also give quite accurate results. You can use the online Ovulation Schedule if you know the date of the first day of your last menstrual period (LMP) and length of menstrual cycle. This will estimate the next fertile period or pregnancy due date based on your last menstrual period.
Calculation of the due date can be done by following a simple mathematical formula known as Naegele’s Rule. This method is used quite often by doctors to determine the EDC. It is an easy calculation to arrive at the likely date of delivery.
- Subtract 3 from the month of your LMP.
- Then add 7 to the first day of the LMP.
Or, to simplify it – (LMP + 7 days) minus 3 months will give the EDC. (If your LMP was on May 10 then the EDD is February 17).
Naegele’s rule is based on a normal 28-day cycle. In women with longer cycles, you have to add 7 days to the first day of the LMP plus the number of days that the cycle extends beyond 28 days.
Sometimes, a woman is uncertain about her LMP and it becomes difficult to calculate her EDD. Ultrasonography is an alternative method of estimating the duration of pregnancy and can give quite an accurate estimate of the EDD, especially when done in the first half of pregnancy.
Measurement of the biparietal diameter (transverse diameter of the head) and length of the femur (thigh bone) can help to determine the fetal age at 20 to 24 weeks. After 30 weeks, the accuracy of measurement by USG becomes less accurate. Fetal crown-rump length can be measured at 5 to 12 weeks and gives a good estimation of fetal age.
The other indirect methods of arriving at an estimated delivery date can be L
An early first trimester examination can give a fairly good estimate of the gestational age.
- At 12 weeks the uterus may be felt just above the pubic bone.
- At 16 weeks the fundus (top of the uterus) lies at the mid-point between symphysis pubis (pubic bone) and umbilicus (belly button).
- At 20 weeks it can be felt at the level of the umbilicus.
- Around 26 to 34 weeks the fundal height correlates roughly with estimated fetal age.
- After 36 weeks the fetal head may descend into the pelvis so estimations become less accurate.
The first perceived fetal movement is known as quickening. It can only act as a rough guide to the age of the fetus and in calculating the EDC since it is felt at different times by different women. It can be felt as early as 14 to 16 weeks by a woman who has been pregnant before, while a first-time mother may not feel the first movements before 18 to 20 weeks.
Fetal Heart Tones
Doppler ultrasound can detect fetal heart beats as early as 10 weeks of pregnancy and is another indirect means of arriving at an approximate due date.
Bony calcification can act as a guide to fetal age, but this method is not used anymore because of the risk to fetus.
Article reviewed by Dr. Greg. Last updated on June 17, 2010