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How can in pregnancy it have 40 wks

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ACOG estimated due date 40 weeks pregnancy first day last menstrual period

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pregnancy gestational age dating

Pregnancy is counted as 40 weeks because doctors start counting from the first day of the last menstrual period (LMP), not from the day fertilization happens.
  • Ovulation and fertilization usually occur about 2 weeks after the first day of the period.
  • So the baby is actually developing for about 38 weeks after fertilization.
  • But because the exact day of ovulation is often unknown, the LMP is an easier standard starting point.
So:
40 weeks pregnant by dates = about 38 weeks from conception.
A due date is only an estimate. Birth can normally happen a little before or after it. The ACOG definition of term pregnancy describes 40 weeks (280 days) from the first day of the LMP as the average length of a singleton pregnancy.

But while we r calculating in negles formula we r taking from lmp only ryt but how can it compare to 40. Wks

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Naegele rule 280 days first day last menstrual period ACOG

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pregnancy dating last menstrual period

Yes, exactly. Naegele’s formula and the 40 weeks are using the same starting point: the first day of the LMP.

Why 40 weeks?

  • From the first day of LMP to due date = 280 days
  • 280 ÷ 7 = 40 weeks

Naegele’s formula

EDD = LMP + 7 days - 3 months + 1 year
This is just a quick calendar method to add 280 days (40 weeks) to the first day of the LMP.
Example
  • LMP: 10 January 2026
  • Add 7 days: 17 January
  • Subtract 3 months: 17 October
  • Add 1 year: 17 October 2026
So the EDD is 17 October 2026, which is exactly 40 weeks from 10 January.
The apparent confusion is that conception usually occurs about 2 weeks after LMP. Therefore, at the time of conception, the pregnancy is already labelled 2 weeks gestational age. That is why a “40-week pregnancy” contains about 38 weeks of fetal development.
Naegele’s rule assumes a regular 28-day cycle with ovulation around day 14, so an early ultrasound can adjust the due date when periods are irregular or dates are uncertain. ACOG’s dating guidance also defines EDD conventionally as 280 days after the first day of the LMP.

But a month have 4 wks only ryt so in 9 months we have 36. Wks and we adding extra 1 wk so 37 wks how came it is 40

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ACOG pregnancy 40 weeks 280 days 9 calendar months explanation

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pregnancy gestational age dating

The mistake is thinking every month = 4 weeks.
4 weeks = 28 days, but most calendar months have 30 or 31 days. So each month is actually about 4 weeks + 2 or 3 days.

Calculation

  • 9 calendar months is about 273-275 days
  • Pregnancy due date is 280 days
  • Therefore: 280 days = 40 weeks = 9 calendar months + about 7 days
That is why Naegele’s rule is:
EDD = LMP + 9 months + 7 days
or equivalently:
LMP - 3 months + 7 days + 1 year
So pregnancy is commonly called “9 months,” but medically we date it as 40 weeks from LMP. The medical text reference defines expected delivery as 40 weeks (280 days) from LMP, versus about 38 weeks from ovulation/conception. - Medical Physiology, pregnancy duration section.
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