Calculating your due date

Your baby’s development actually takes 10 lunar months. (Although generally, people always talk of a ‘nine-month’ pregnancy) A lunar month consists of exactly 28 days.

A full-term pregnancy, therefore, lasts for 40 weeks (280 days) from the first day of the last menstrual period (LMP). However, this calculation assumes a cycle length of 28 days. If your cycle is not 28 days, you will have to calculate from an adjusted LMP date, that is, an LMP date adjusted as if you did have a 28-day cycle.

(This calendar, thank goodness, does this calculation for you automatically).

But, if you really enjoy figures, and you want to check, you can calculate your own adjusted LMP.

The LMP date + (Your cycle length - 28) = Your adjusted LMP date.
Example:
If your LMP is August 22, with a cycle of say, 32 days
Your adjusted LMP will be 26 August. Why?
Because (32 – 28 = 4 days) & 22 + 4 = 26 August. Easy!
Or:
If your LMP is 22 August with a cycle length of 24 days this time.
The adjusted LMP will be 18 August. (24 – 28 = minus 4 days) Get it?

This LMP adjustment will allow you to calculate your own due date using the standard tools. Alternatively, you can calculate your due date using the standard tools, and then adjust your due date as described below.

How "far along" you are can be calculated in two ways:

  1. From the date of your LMP;
    OR
  2. From the date of conception, which typically occurs two weeks after your LMP, which is called weeks from gestation.

When's my due date?

The traditional way to calculate a due date is to add 7 days and subtract 3 months from your LMP date (adjusting the year if necessary)
For example:

  1. You have a LMP of 21 May 1999 (21/05/99).
  2. Add seven days, bringing you to 28 May (28/05/99).
  3. Subtract three months, bringing you to 28 February (28/02/99).
  4. Finally, adjust the year to the next year 28 February 2000 (28/02/00).
  5. Thus, your due date is 28 February 2000 (28/02/00). Well done!
Because women get pregnant in all the months of the calendar, some of which have 28, 29, 30 and 31 days, calculation of due dates is rarely precise. As a result, a baby is considered full-term three weeks before the estimated due date (EDD).

But if you calculate my due date from LMP, you are counting weeks before I even conceived.

Although it may be confusing at first, health care providers begin counting the pregnancy from day one of the LMP. Yes, before you were even pregnant. They do this to make up "lost days" -- that is, because the typical cycle averages 28 days, several days at the end of each calendar month appear to be lost. These "lost days" are compensated for by the two weeks at the beginning of your cycle, prior to conception, and thus make up the extra month in the tenth lunar month of pregnancy. We know it is confusing.

Making matters even worse, many women do not have typical 28-day cycles. Practitioners may adjust the EDD to reflect the shorter or longer cycle length. Here are two examples of how this is figured:

  • Jenny's cycle is regularly 24 days, approximately four days shorter than the typical 28-day cycle that her practitioner uses to determine the EDD. Jenny will subtract four days from her EDD of 27/01/99. 27/01/99 - four days = an adjusted EDD of 23/01/99
  • Wendy's cycle is 35 days, approximately seven days longer than the typical 28-day cycle. Wendy will add seven days to her EDD to get an adjusted due date
In addition to all that hampering an accurate due date calculation, some women may be unable to recall their LMP. If this happens to you, recalling past events such as family birthdays, holidays or vacations may trigger your memory. When you visit your midwife or doctor for the first time, try to give as close an estimate as possible. The closer the estimate, the less likely both you and your doctor will worry or experience stress if your baby does not come on the approximate due date. (Only about five percent of expecting mums deliver on their designated due date. So, don’t worry).

Occasionally, women with irregular or infrequent cycles cannot accurately identify their LMP’s. An experienced practitioner may have to rely on physical clues to determine the baby's due date. Most of these clues are most evident within the first two months of pregnancy. They include:

  • Examination of uterine size
  • Identification of audible foetal heart beats by doppler and/or foetal stethoscope
  • Ultrasound examination prior to 26 weeks from LMP (Note: The EDD may be off by as much as two weeks in either direction.)
No matter what method is used to determine EDD, the final factor that can disrupt the best estimated due date is your baby, who will choose his or her own birth date regardless of anybody.