Should a woman become a surrogate if she's against having an abortion?

I'm not going to make this case specific. It doesn't matter what the disability is, really. If a surrogate is hired, then the patrons find out that the baby has a disability that they would have aborted if it were their biology at play but she refuses...?

From this article on Slate:

Roughly 2 percent of all pregnancies are complicated by a major birth defect, and of those about 0.5 percent have a chromosomal defect, such as an extra or missing segment of normal DNA. Birth defects are a leading cause of infant mortality, and in many cases of severe birth defects, no medical treatment can salvage a fetus's life or result in any measure of normal future health.

I mean, obviously it couldn't be built into law that surrogates must be okay with having an abortion should their patrons request it, but could it even be something put into a contract?

I know it goes without saying, but I'll say it anyway: fertility and abortion are sensitive topics 'round these parts so don't be a butt.

ETA: I guess I'm looking for an ethics-based discussion. I know you can put practically anything in to a contract, but can you really?