Due Date Calculator: iVillage Pregnancy & Parenting


What is a Lunar Month?

Prenatal development is typically measured in lunar months. Each lunar month consists of 28 days, organized into four weeks of seven days each.


Health care providers begin counting the pregnancy from day one of the LMP. Yes, before you were even pregnant! Health care providers 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 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:

  • Jenna's cycle is regularly 24 days, approximately four days shorter than the typical 28-day cycle that her practitioner uses to determine the EDD. Jenna will subtract four days from her EDD of 1/27/05. 1/27/05 -- four days = an adjusted EDD of 1/23/05.
  • Nora's cycle is 35 days, approximately seven days longer than the typical 28-day cycle. Nora 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 care provider for the first time, try to give him or her as close an estimate as possible. The closer the estimate, the less likely both you and your practitioner will worry or experience stress if your baby does not come on the approximate due date. Did you know that only about five percent of expecting moms deliver on their designated due date?


Occasionally, women with irregular or infrequent cycles cannot accurately identify their LMPs. 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 fetal heart tones by doppler and/or fetoscope.
  • 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, there is a final factor that can disrupt even the best estimated due date. That is, ultimately, your baby will choose his or her own birthdate!

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