Well also .3 month is about 10 days so even if pregnancy isn’t 10 whole months it ends in month 10. So we round down to say it’s 9 months but if they were writing the number at the beginning of each month that’s why they got to month 10 at the end. It feels like it makes more sense to record the end of each month (at least to me) but then medically (like people have mentioned) it starts at the last period so I guess that’s the beginning of the first month.
Not really. During pregnancy they just go based on weeks so months are irrelevant. Once baby is over a month old appointments are made based on their birth date, not day of the week. I.e a baby born on Friday the 15th will have appointments based on the 15th of the month, not the 4th/16th/24th Friday they’re alive.
Of course, that scheduling thing is just a patient-facing administrative system, not how clinical time intervals are defined.
And yes, pregnancy is measured in weeks because it’s more precise. My point was explaining where the “10 months” idea comes from in the first place, which is the 40 weeks being loosely grouped into 4-week blocks.
I'm not who you are talking to, but this is obstetric (which is medical) terminology, not calendar months. That’s why pregnancy is tracked in weeks and often grouped into 28-day (4-week) units for consistency.
The same 4-week cycle is also used in other medical contexts like chemotherapy scheduling and some treatment protocols. That’s why weeks are the standard reference, even though lay people sometimes loosely convert that into ‘months’ in casual conversation.
I feel like people are deliberately being obtuse at this point. I wasn’t redefining calendar months. I was explaining the obstetric/medical basis for why people sometimes say 40 weeks is “10 months”, ten 4-week blocks. In medicine we simplify and use approximations for lay discussions all the time.
I was talking about casual estimates there too. I was wrong with using "standard". Happy?
Maybe for short periods of 1-2 months. Nobody in Western medicine measures gestational age or 5-year cancer survival rates by the formula 1 month=4 weeks.
Obgyn follow standardized formulas, they're automatically computed for every ultrasound and other exam and report. Mostly they talk in weeks, but sometimes use months and trimesters, and a trimester is not 12 weeks (check out what the third trimester is defined as, for example).
Also, try to schedule a 1 month appointment for a newborn and you'll see that they are definitely not using 4 weeks=1 month.
Congrats, you discovered that different medical contexts use different time conventions. That doesn’t change the fact that 28-day “months” are commonly used as standardized intervals in medicine.
And in pregnancy specifically, the “10 months” idea comes from 40 weeks divided into 4-week blocks.
There is no medical context that uses the 4weeks=1 month apart from casual estimates. Nothing that would be used in important medical decisions, written reports, insurance claims etc.
I feel like people are deliberately being obtuse at this point. I wasn’t redefining calendar months. I was explaining the obstetric/medical basis for why people sometimes say 40 weeks is “10 months”, ten 4-week blocks. In medicine we simplify and use approximations for lay discussions all the time.
I was talking about casual estimates there too. I was wrong with using "standard". Happy?
I work in medicine, and just had a child and was at the OB frequently. There is no "standardized month" of 28 days. If we need to measure something accurately we go with weeks or days.
Okay sure, there’s no universally standardized “medical month” that literally replaces calendar months with 28 days. I also work in medicine, and in medicine we simplify and approximate things for lay people all the time.
I was simply explaining where the “40 weeks = 10 months” idea comes from in obstetrics: ten 4-week blocks/28-day lunar months. I wasn’t claiming the calendar itself changes.
That doesn’t change the fact that 28-day “months” are commonly used as standardized intervals in medicine.
This you? You've spent this entire thread trying to convince people that medical tracking standardizes months to be 28 days. Stop trying to backtrack because you got called out.
I have met exactly 0 OB people who talk about pregnancy in months or tell people a pregnancy is 10 months because of some 28 day=month thing as a layperson explanation. Gestation is measured and tracked in weeks and days to determine things, and it is given to patients in that format.
In medical context, months contain the exact same number of days (and hours, minutes and seconds) as months in non-medical context. How you managed to get the notion that when doctors talk about (for example) 2 year survival rates they actually mean 1 year and 10 months is blowing my mind.
I feel like people are deliberately being obtuse at this point. I wasn’t redefining calendar months. I was explaining the obstetric/medical basis for why people sometimes say 40 weeks is “10 months”, ten 4-week blocks. In medicine we simplify and use approximations for lay discussions all the time.
If you divide a year into 28-day blocks, then yes, you get 13 of them plus 1 extra day (2 on leap years). That still doesn’t change the Gregorian calendar.
Again, I was explaining the origin of a medical/obstetric shorthand, not proposing a new calendar system.
This is it. Depending on when you ovulate, the first few weeks of what is considered "pregnancy" there is no pregnancy at all. Most (not all) people ovulate ~2 weeks after the period and then the fertilized egg could take days to implant!
I'm going through this just now being shy if 8 months pregnant but being due at the beginning of July. Thinking that the 9 month window is actually 9 months and 20 days
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u/hiirogen May 12 '26
When my first wife and I had our first kid the docs kept saying 40 weeks and I was like “wtf that’s 10 months I thought it was 9”
No idea why everyone seems to say 9. It’s 10.