How many kids could we have? (Or, "Why I'm impressed with the Duggers")

In my last post I mentioned that NaturalGuy and I have a great deal of confidence that we couldn't have 20 kids if we tried. I thought I'd share our math.

We're young - 23. The average age women reach menopause, defined as a full year without menstruation, is 51, and almost all women reach menopause by 55. It's worth noting that fertility declines significantly before this. But, let's be generous and assume I'll have 30 fertile years.

Pregnancy is 9 months, so if back to back pregnancies were possible, I could be pregnant 4 times in 3 years, leading to 40 children.

But breastfeeding reduces fertility. The average user of ecological breastfeeding has a little over a year before her fertility returns. So, combined with pregnancy, 12+9=21 months of infertility. That alone reduces the number of kids we could have to a little less than 20.

All 30 of those fertile years are not equally fertile either. Let's suppose that 20 of those years are quite fertile. If I had a baby every 21 months until I was 43, I'd have about 12 kids. Then, suppose from age 43 to 53 it took twice as long to get pregnant, getting pregnant shout every 3 years - that's another 3 kids. This gives a total of 15 kids - and that's assuming that we relied exclusively on breastfeeding to space our kids.

15 kids seems like about 10 more kids than we could support, but as a maximum, assuming we never used NFP to postpone pregnancy and had exceptional fertility as we aged, I think it is interesting. Even if we didn't try to limit our family size, there would be a natural maximum that we'd have trouble exceeding. It makes me wonder if there's any data on the topic...