If pregnancy is rounded off to 9 months then why do doctors give your due date on the 40th week/10th month?

in school, you always learn women are pregnant for 9 months

but then why do doctors say your due date is your 10th month? going off a 4 week month....

and many women give brith past due date.


when some of you are telling me its because its the whole nine months, im not understanding what your saying...

15 Answers

  • 1 decade ago
    Favourite answer

    Reading the responses you've gotten so far actually confused me, and I know the answer to your question!

    It has nothing to do with not knowing you're pregnant for the first month.

    Pregnancy is measured in weeks from the first day of your last menstrual period (LMP). Average gestation for humans is 40 weeks, with 37-42 being considered "normal."

    A month is not 4 weeks, except February, sometimes. 4 weeks would be 28 days, and we all know a month is usually 30-31 days. These extra days add up.

    So... I am pregnant now so I will use myself as an example. My LMP was on 10/20/08 and my estimated due date (EDD) is 7/27/09. If you're counting in months, this means I'm due 9 months and 7 days after my LMP... 40 weeks. If months were counted as 4 weeks, you'd be right and gestation would be considered 10 months, but because months are not counted by weeks, but are instead counted by days, it doesn't work that way.

    People say "9 months" because it is more easily grasped than saying "40 weeks."

    As for women going past their due date, this all depends on your genes, your body, your baby, and a slew of other biological factors. The average gestation for a first-time white mother is 41 weeks and 1 day. The average hispanic woman has her babies earlier and the average white woman who has had babies before goes earlier than the 41 weeks, 1 day time also. If your mother and her mother both had their babies earlier or later than their due dates, you can expect the same, though it's not a guarantee. After all, the father's genes also make a difference, as does your physical build and how fast the baby is ready.

    This is why a woman is considered "full term" at 37 weeks, even if the due date is placed at 40 weeks... because it is normal and healthy for many women to give birth as early as 37 weeks naturally. However, some babies are not ready at 37 weeks, meaning that if a woman has her labor induced at this time, the baby could have breathing problems and other signs of prematurity. All babies are different in their development, just like after they're born.


    37-42 weeks is considered "normal" gestation.

    People say pregnancy is 9 months because when counting the actual days and weeks to "average" gestation, it is closer to 9 months than 8 or 10 months. In all reality, "average" gestation is 40 weeks.

  • 1 decade ago

    Is a pregnancy nine months or ten months long? This is a popular pregnancy debate with many women across message boards and in pregnancy communities today. Most people know that a pregnancy consists of the first, second and third trimester. The definition of a trimester is “A period or term of three months”. If this is the case, why do so many people consider themselves pregnant for ten months?The answer is simple, even if not actually correct. Pregnancies are considered full term at 40 weeks, and many women count four weeks as a month while they are pregnant, making the result a ten month pregnancy. On average there are actually 4.33 weeks in a month, however that a third of a week tends to get lost in the pregnancy shuffle for much of us.

    Many women also find it easier, when asked how pregnant they are, to answer in weeks instead of months, because pregnancy is generally a count down. “I am 24 weeks, only 16 weeks to go!” The most general deduction by a bystander would be that the woman is already six months pregnant, when in actuality, per the Gregorian calendar that most of the world lives by, she is only about five and a half months pregnant.

    Women who consider themselves 10 months pregnant usually use this form of calculations:


    1-4 1

    9-12 3

    17-20 5

    25-28 7

    33-36 9

    For those that live by the Lunar calendar, this not only makes sense, but is absolutely correct. The Lunar calendar, which is the four weeks it takes for the moon to go from a new moon, to a full moon and back again, would make a pregnancy last for ten months.

  • Lilli
    Lv 7
    1 decade ago

    You are pregnant for 9 months in the calendar. But a month is not 4 weeks (except February). So every month, you are pregnant for a little more than 4 weeks, which adds up to an extra 4 weeks, making it 40 weeks total.

    You are not pregnant 10 months on the calendar, but you are pregnant for 40 weeks.

    Your question says "but then why do doctors say your due date is your 10th month? going off a 4 week month...." You answered yourself - they are going off of a 4 week month, which is not actually what a month is on the calendar.

  • Stacey
    Lv 6
    1 decade ago

    There are more than 4 weeks in every month.

    If there was only 4 weeks in every month, there would be 13 months a year.

    The 40th week is technically at the end of the 9th month and into the 10th, so you are pregnant for 9 FULL months.

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  • 1 decade ago

    Pregnancy lasts 40 weeks, almost 10 months. BUT for the first month of pregnancy you do not know you are pregnant, therefore most women know they are pregnant for about 9 months.

  • 5 years ago

    The answer is actually much simpler than any of these. The forty weeks includes the first two weeks of your cycle, your period and implantation. You're not pregnant during those two weeks which is why even though you deliver at forty weeks (on average) you're only pregnant nine months.

  • Anonymous
    1 decade ago

    You answered your own question when you said "pregnancy is rounded off". When they say nine months, they are rounding it. Also, it is meant to be an average since, as you also said, some women go past it.

  • 1 decade ago

    Its a full 9 months so u dont give birth in the begining of the 9th month u give birth at the end of the 9th month

  • 1 decade ago

    Because every woman is different and some give birth later or earlier than others. It's actually only 38 weeks because they include the two weeks before you ovulated so you actually weren't even pregnant yet.

  • 1 decade ago

    Because months are NOT 4 weeks long.

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