Plan
If you're talking about US surrogates, perhaps you should try reading a standard Californian surrogacy contract? Basically all rights to her own body are taken away from the mother.
If the buyers decide that they want to abort, she has to agree or pay back all the costs so far - IVF, check ups etc. This could be aborting for medical reasons, because it's twins and they want to do selective reduction, or any reason they choose.
To a certain extent the buyers actually get a say in any medical decisions that aren't pregnancy related - there's a podcast about a surrogate mother who was told by her doctor that being on the hormones was life threatening, but the buyers still wanted her to stay on them.
A PP mentioned Kim Kardashian as needing surrogacy: well if it's such a fantastic thing to do, why did none of her sisters step up and offer? Could it be because it's the rich taking advantage of the poor?
And has everyone forgotten cases such as Baby Gammy, abandoned because he wasn't perfect?