We have two lovely boys. 9 and 6.
Life has had its ups and downs (more downs) over the last few years, it has been really shit for so many reasons. Not our relationship, it is good. But he wants a third child, and bring this up gently ever so often.
I give my reasons why not. And he listens.
Like:
The age gaps will be big.
Not fair on our boys that I will be pregnant and out of action for some time, considering we like to go mountain hiking, cycling, camping.
Travel abroad will be more expensive with a third child.
Need bigger car
Secondary school fees for one extra child.
Expensive
etc.
All very shallow reasons, I must admit.
The biggest fear is SPD again.
I feel "too old " for sleepless nights, breastfeeding, etc again.
Worried PND will rear its ugly head.
Non of these reasons are good enough for somebody who wants another child, though. 
I am torn. I am also 40.