I know nothing about equine prenatal care, but wanted to gently disagree that when a human is pregnant, the providers are only checking that she is still pregnant, and nothing more. Or at least, if that IS all your providers did, you should have fired them.
They check for many conditions that can be caused by pregnancy, including gestational diabetes and pre-eclampsia, monitor the size of the baby (too big can indicate health problems in the mother and potential problems with delivery, too small can indicate the baby isn't getting enough nutrients from the mother, among other things), the position of the baby, the amount of amniotic fluid, and yes, deformities and other problems with the baby that can be detected by ultrasound and other methods. And yes, some of those problems don't have any good solutions, but some of them can be helped pre-natally (they even do surgery on fetuses in utero!), some can be prevented from getting worse, and some may not be preventable or solveable, but just knowing about them can mean having the right medical staff on hand when the baby is born.
I'd imagine it's roughly the same with equine pregnancies. In a normal healthy pregnancy, it probably doesn't SEEM like the vet care makes much difference, but that just means things are going well. And if there IS a problem, much better to catch it early, prevent it or mitigate it when possible, and be prepared for the inevitable when necessary.