Hi L.,
My sisters are 5 and 7 years older than I am and I never liked it. They always had each other, I was "alone". (I always have shared a bedroom with one of my sisters till I was 12, and they were really nice to me, but still...) esp. when I was 10 - 15 years old, and they had left the house for college and I was alone with my parents...
My mother in law got pregnant when she was 39, and my 'little brother" has Down syndrome. So: never under estimate the risks when you're getting older. (I love my little brother though)
So far the negative part.
About family dynamic. In my opinion going from 2 to 3 children is "easy". But going from 1 to 2 is such a difference. One child you can take everywhere with you. You can let him sleep on a sofa or on the floor wherever you are, never a problem. But with 2 you can not do that anymore, so you have to go home for naptime. With one I was never on a schedule, he was just on my schedule. With 2 I was always on a schedule, always planning. When nr.3 came, we were already on a schedule, so it was "easy" to fit his schedule in the big family schedule. (don't know about when they will be 9,7 and 6 and they all play soccer at different times... must be hard to have a schedule then. I hope i will have learned it by that time)
So if you think 2 is easy, 3 will be easy for you too. And 3 kids still fit in one car. (With 4 you have a problem with that.)
You have to figuere it out for yourself (succes!), these are just a couple of thoughts...
Don't know about the thin uterus.
Take care, I.
(3 boys, 4,5 year, 2,5 year, 1 year)