Possibly women who are smarter are more likely to have interesting and challenging jobs and higher incomes (note: generally, not always) and may believe they have more to lose by having children
So I can only comment on my own situation. I'm 37 in a couple of weeks and we haven't started TTC yet though we really need to.
I'm the main breadwinner by quite a long way compared to my DH. I met him later and so we've had the expense of a wedding and first house in the past 18 months.
We really want to TTC but I'm a freelancer/contractor and so have no enhanced maternity pay. We have savings but only enough for me to take max 4 months off on mat leave.
My main worry though is that I have a contract due to finish between end October and end of year and I won't get hired if I'm obviously pregnant. As I work through a limited company I have no protection from discrimination on the basis of me being pregnant. This could then potentially use up all our savings if I'm out of work which would then mean I have to give birth and get a job straight away. Added on top of all that I have bipolar disorder and so expect the hormones/sleep deprivation to be relatively hard on me.
There's no point in me going back to perm roles as in my industry you normally have to do 2 years before qualifying for enhanced maternity pay.
Complex situation but in summary - because I'm the breadwinner it's a lot more difficult for us to have children.
If DH could get pregnant we'd probably have two by now!