Pregnancy Length

Weeks? Months? Trimesters? Lunar months? Days?

The date of conception is not always known and can vary between women. The date of LMP is known by most women so doctors use this date as a starting point for pregnancy. From your LMP they will add 280 days and this will give your Estimated Due Date or EDD. In general, all doctors agree that pregnancies, when using the LMP dating technique, average approximately 280 days or 40 weeks.

Week by week pregnancy information table

9 months or 10 - Confusion starts here.

Timeline breakdowns for pregnancy

• Days - 280 days from your LMP (Gestational) or 266 days from conception to birth (Fetal age).

• Weeks - 40 weeks from the start of your LMP (Gestational).
               38 weeks from conception to birth (Fetal age).

  Most doctors count pregnancy weeks from LMP (Gestational), a few others may count from conception to birth (Fetal age); and the week numbers may vary.
  Example: Week 28 (Gestational) = Week 26 (Fetal age)

  One thing to note is - four weeks from the first day of your last period you are considered "4 weeks pregnant", but, "in week 5 of pregnancy". This can get confusing, especially considering that your baby is actually, only about two weeks old.

• Months [ Calendar ] - Measured as nine months on the calendar + extra 7 days for Duedate. According to the Gregorian calendar, where months range from 28 - 31 days, this means the average pregnancy lasts a little over nine months.

  Examples: Whichever one is easier for you -

  Example 1 -- starting one week after your LMP
  LMP = January 1st + 7 days = January 8th   + 9 calendar months = October 8th [Duedate]

  Example 2 -- add one week on the end
  LMP = January 1st + 9 calendar months = October 1st  + 7 days = October 8th [Duedate]

  Note: Always count the full months from the first day of your last period, like you would count the number of years in your age ie. in your first year you were 0, in your second year you were 1 year old, etc.   eg. LMP = January 1st ~ on February 1st you are 1 month pregnant, on March 1st you are 2 months pregnant etc.

    Interesting triva:  3 calendar months = 13 weeks.
    Go on, try it!
  Take a calendar, pick any date, count forward 13 weeks, this will get you to 3 calendar months (minus a day) from where you started.

  eg. July 1st + 13 weeks, gets to September 30th, 1 day short of the 3 calendar months. Pick March 10th, + 13 weeks, gets to June 9th, again 1 day short of the 3 calendar months.

  - if the months you choose includes February, you get 2 days short of the 3 calendar months, except leap years, when again it is 1 day.

  - Count out your 40 weeks of pregnancy on a calendar, it's exactly 9 calendar months + 1 week.

• Lunar Months [ True ] - A true lunar month is the time between two successive new moons or full moons. Lunar months mean length is approximately 29.53059 days (29 days, 12 hours, 44 minutes and 3 seconds). 280 days Gestation is approximately nine and a half true lunar months, since a lunar month is approximately 29.53 days long.

• Lunar Months [ Pregnancy ] - In some countries (mainly Far-East) pregnancy is counted using lunar months of exactly 28 days (i.e. 4 weeks), then 40 weeks = 10 months.

  In Japan, the normal length of a full-term pregnancy is considered to be 10 months, and not 9. The Japanese belief that a pregnancy lasts ten months refers back to their original calendar that used these shorter months. To confuse things further, the classical phrase used in Japan for a pregnancy is 'totsuki tooka', which refers to a length of ten months plus ten days.

  Some Asian countries in the Far-East did not incorporate the Gregorian calendar until the late eighteen hundreds, the Chinese did not start until 1912.

• Trimesters - From Latin trimestris; from tri- ("three") + mestris ("months") 'three months'.
  Three periods of three months each. Months 1, 2 and 3 are the first trimester, months 4, 5 and 6 are the second trimester, and months 7, 8 and 9 are the third trimester.

  Most commonly, the first trimester is defined as from LMP through to end week 13.
  The second trimester is from week 14 through to end week 26.
  And the third trimester is from week 27 through to labor/delivery, which varies considerably but averages at week 40.
  13weeks + 13weeks + 13weeks = 39weeks   + 1week = 40weeks

• Beginning
  In the first trimester the embryo undergoes most of its early structural development. Most miscarriages occur during this period, the placenta will takes over hormone production around week 12 from the corpus luteum. Sometimes, the failure of the corpus luteum to adequately support the pregnancy with progesterone can result in an early pregnancy loss.
• Middle
  In the second trimester the embryo, now known as a fetus, is recognisable as human in form, but is not developed enough to be viable if born. The first three months were a period of rapid development, the middle three months are a period of rapid growth.
• End
  In the third trimester the fetus reaches viability, and may survive if born prematurely.

Even an estimated due date is nothing more than a estimate. Only 10% of women deliver on the estimated due date. Half of women hit the due date within one week, but 90% deliver within two weeks of the predicted due date. This is why some obstetricians have started predicting 'due weeks' instead of 'due dates'.

Your 'date' may be revised by health care providers during your pregnancy. Your baby's development is on a strict schedule during the first trimester. The fetus CRL (Crown to Rump Length) and the EHR (Embryonic Heart Rate) are accurate estimators of fetus age in the first trimester. CRL and EHR are measured at your ultrasound scans, the fetus age should be within + or - 1 week of your LMP.

No matter how you measure your pregnancy, even the best estimated due date can be inaccurate. So, although the waiting and uncertainty of not knowing exactly when your baby is to arrive can be unnerving, the best plan is to stay flexible and to always be prepared for the unexpected.

Q: This all sounds fine, but if you calculate the due date from LMP, are you counting weeks before conception?

A: Yes a Pregnancy lasts about 40 weeks from the start of your LMP, and is 38 weeks from conception to birth. Although it may be confusing at first, health care providers begin counting the pregnancy from day one of the LMP. Yes, that is about 2 weeks before you even conceived.

Physicians and midwives start the pregnancy clock on the first day of a woman's last menstrual period (LMP). Some women may be unable to recall their LMP or 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 you will worry or experience stress if your baby does not come on the EDD. Only about ten percent of expecting moms deliver on their EDD anyway.

An experienced practitioner will also rely on physical clues to determine the baby's due date. These clues are most evident within the first few months of pregnancy and include:

• Examination of uterine size (Fundal Height)
• Identification of audible fetal heart tones (Week 6 Fetal development)
• Ultrasound examination prior to 26 weeks from LMP (Dating scans)

So when is my due date?