This is what got me to thinking about it in the first place. [Were the old movie depicting fact the couple were probably having coital interactions four times a night from the time his draft lottery number came up and the time he was mustered.]
The " … So, and I may not be remembering my statistics right 7/28 = .25 * .25% = 6.25% … " makes the most sense to one knowing virtually nothing about it. Naturally assumptions about age and health and that sort of thing must be assumed for such guestimating.
But all you’ve shown is that the probability of doing it on a fertile day, if the day’s randomly selected, is one in four. (More like one in three given that seven out of 28 days are likely to be deselected for an obvious reason.) It does not therefore follow that every time you do the nasty on a fertile day, conception always results. And the actual figure (I’ve also heard 85% as the figure quoted for the annual chance of pregnancy if you’re regularly having unprotected sex) strongly suggests that it doesn’t.
The numbers I’ve heard are that sperm reliably survive for 3 days and an egg reliably survives for 24 hours, leaving 4 fertile days a month, so I’m going back to this thread’s original 1/7 or 14% estimate.
Studies of chemical pregnancies and early miscarriages suggest that 40% of fertilized eggs never become happily ensconced zygotes. 60% of 14% is 8%. Furthermore, there is about a 10% chance of miscarriage after the early weeks, taking us down to 7.2% chance of successful pregnancy from one random sexual act per cycle.
Though as has been mentioned above it’s probably a little higher than this due to lesser likelihood of nookie during menses and a higher likelihood during fertile days.
Especially since I believe there have been studies that show that women are more likely to be in the mood in their fertile period. I do know for sure there have been studies that show that a woman is more likely to dress in what she considers sexy in those time periods.
Just because we can have sex at any time doesn’t mean that some of the natural cycle isn’t present. It only had to be strong enough to make babies more likely than not.
If that’s the period, most women whose periods I’m familiar with (relatives, former classmates, coworkers) don’t have them last that long; 5 days is very common, and many of us only get heavyish flux for 2-3 days, the other days are stop’n’go.
From here, but the actual number of miscarriages may be higher as many women who have chemical pregnancies never realized they were pregnant. I have a friend who only discovered a miscarriage because she was taking a pregnancy test every night. Most women aren’t that vigilant, and a lot of miscarriages aren’t reported to medical professionals.
Well, since we’re anecdoting and all, I’ll just say that having been married for 18 years and in a couple of long-ish term relationships before, I didn’t feel I needed the correction.
Scrolling down to read new posts to this thread, I realize I forgot to tell you how much I love this response. Kudos.