The number one question is whether she'd be absolutely willing to let you attend the entirety of every single medical appointment and get every single medical record you feel is necessary to see regarding her (regardless of whether related to the pregnancy). And to be clear, say under the contract, if she refuses to hand over anything, then any right to further compensation will be immediately cut off. Demand total transparency, and see how she reacts.
Our own surrogate made it seem like she would be transparent, and even signed a contractual provision requiring her to be transparent -- but it was a bald-faced lie. She ended up refusing to allow us to attend medical appointments and refused to share medical records until our child was born premature at 26 weeks (see my prior posts).
Shortly after our baby was born, we threatened to sue her if she didn't hand over the full medical records, and she finally did so. According to the doctor, our baby was born early apparently for reasons outside her control, but still, it was the most harrowing experience of my life to not know if the reason this happened was because of some secret she was keeping. In the end, I think she didn't have anything to hide exactly, but was just fucking nuts and paranoid about sharing any information. The psychological exam we did of her beforehand didn't even so much as hint at the problem. And all this despite the fact she had done surrogacy before, apparently without a problem.
She's honestly one of the most evil people I have ever had the misfortune to deal with -- I'll never forgive myself for not grilling her more before this all happened. In practice, the surrogacy agreement isn't even worth the paper it's written on once she's 5 months pregnant, at which point your surrogate has all the leverage and can basically do whatever she wants (and you have to live with it).
Look her in the eyes, grill her, make her a bit uncomfortable, and decide for yourself if you see any hint of her being a fucking liar. That is my advice. Don't buy into the touchy feely bullshit that the "match call" is supposed to be some sort of bonding experience. I'm glad to say our son is now about 6 months old (3 months adjusted) and healthy, but regardless, the surrogacy business has more than its share of frauds and bad actors, and you cannot be too cautious. Buyer beware.
One other thing -- make sure she's willing to pump immediately after delivery for breast milk, and get an agreement to that effect (which can be enforced via a TRO and specific performance). If your child is born severely premature, then the best thing for him or her to get will be her breast milk. For whatever reason, when children are born premature, the mothers produce a very rich form of breast milk that is very good for the babies. Breast milk from milk banks isn't nearly so rich, since it's typically from mothers who gave birth full term. Our own baby struggled with growth in the NICU, and I have no doubt this is because we weren't able to get breast milk from the surrogate. Nobody told us breast milk was so important for premature babies until nearly a week after our baby was born, long after it was too late for the surrogate to start "expressing" and potentially providing breast milk.
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Edit: Seriously, who downvoted this? I'm literally just giving you my fucking reality as a NICU dad.