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Months, Lunar months, Weeks, Days & Trimesters

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If you are confused, you're not alone. While most people talk about pregnancy being 9 months long (divided into three trimesters), most health care providers refer to a pregnancy as being 40 weeks long, starting with your last known menstrual period. You might like to know that this is also equal to 280 days, or 10 Lunar Months. Phew!

OK, so when is my due date?

The traditional way to calculate a due date is to add 9 months from the first day of the LNMP (last normal menstrual period) and then add seven days.

For example:
1.Your LNMP was Feb 1
2.Add nine months, bringing you to November 1.
3.Add seven days, bringing you to your due date of November 8
4.Thus, your due date becomes November 8!

Some people prefer to do it this way:
1. Your LNMP was Feb 1
2. Add 1 day
3. On your calendar, count forward 40 weeks to determine your due date. 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.

Remember, calculation of your due date is certainly not precise (after all, women get pregnant in all the months of the calendar, some of which have 28, 29, 30 and 31 days!). Also, all women do not have a 'typical' 28 day cycle! This is why your due date is usually referred to as your EDD or Estimated Due Date!

As if it wasn't already hard enough, some women may be unable to recall their LNMP or may have very irregular or infrequent cycles. When you visit your care provider, try to give 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 EDD. Only about five percent of expecting moms deliver on their EDD anyway!

An experienced practitioner may also 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
Ultrasound examination prior to 26 weeks from LNMP

This all sounds fine, but if you calculate the due date from LNMP, aren't you counting weeks before conception?

Although it may be confusing at first, health care providers begin counting the pregnancy from day one of the LNMP. Yes, before you even conceived!

Months
Measured as nine months on the calendar -- starting one week after your LNMP.

Trimesters
Three periods of three months each. Months 1 - 3 are the first trimester, months 4 - 6 are the second trimester, and months 7 - 9 are the third trimester.

Lunar Months
Prenatal development is often measured in lunar months. Each lunar month consists of 28 days, organized into four weeks of seven days each. That means a pregnancy is 10 lunar months long!

Weeks
40 weeks from the start of your LNMP. Gestation is 38 weeks from conception to birth.

Days
280 days from your LNMP. No matter what method is used to determine EDD.

No matter how you measure your pregnancy, even the best estimated due date can be inaccurate. Your baby will be born when he or she is ready!

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Guest Oct 11, 2016 05:17:01 AM ET

This post has resolved our problem. Thank you very much and hope you write more good articles.

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