I think it is irresponsible of a doctor to not mention when termination is an option; but I agree that doctors need to make sure patients understand the options to carry to term and get referals to get information to care for a potentially disabled child after they are born. Do you mind me asking- you say "diagnosed", did your child not have Downs syndrome? Or is it only because an ultrasound is not diagnostic?
In my case, when I found out my child was incompatible with life (they suspected trisomy 13 or 18; but it ended up being an incredibly severe chromosomal deletion) the doctor told me "It is unfortunate you are past the point of time where it is legal to terminate in our state. If you want to terminate the pregnancy, here are two clinics in the US that could do it. I wouldn't judge you if you choose to get on a plane tomorrow, but I'd recommend you get the genetic testing done so you have some answers about exactly what is happening; and we can better determine if the life expectancy is a few hours, a few months, or a few years. If you'd like to do that, in the meantime we can make seperate appointments with the pediatric cardiologist, urologist, neurosurgeon, plastic surgery team, nephrologist, opthamologist, and gastroentologist who will be part of your son's management team in the NICU and first year of life if he is born alive. In a few weeks, we will talk about convening a panel to talk about life-"
That conversation could have gone either way "well, it's too bad your previous OB never mentioned anything was wrong until 25 weeks, what a weird coincidence, can't terminate" or "wow- you have no choice but to fly out of state and terminate this pregnancy, there is absolutely nothing we can do for you." or "oh man, that sucks- this baby is gonna die. Well, see you when you deliver."