I’d like to expand on my thoughts on necessary vs elective surgeries:
One of my children has undergone 3 major surgeries: The first, at 4 months of age, was open-heart surgery to fix her heart (she was born with multiple deformities of the heart which would have killed her eventually. But they put it off as long as they could, until her health began to fail). She later had a G-tube placed at 6 months of age, and a nissen fundoplication/pyloroplasty done at 15 months of age. All of these were absolutely necessary to sustain life or quantifiable quality of life. Today, she is a healthy child with a lot of scars. We had to submit her to the surgeries, or lose her, or subject her to a life so miserable it would have been almost impossible for her to be normal or healthy I am not opposed to necessary surgeries, and we understood the risks inherent to general anaesthesia, not to mention possible long term side-effects of the surgeries themselves (fundoplication itself can cause long-term retching and gagging, decreased motility, and the inability to belch up excess gas).
In our case, it was not hard to choose surgery, because we no other realistic choice. But anaesthesia is dangerous for babies. This is why they try not to do surgery on babies unless it’s unavoidable. And this is why routine infant circumcision, which by any objective scientific standard of measure (like cortisol levels in the blood) is excrutiatingly painful, is done without anaesthesia at all, or at best with a local block which is only partially effective. This is for the child’s safety. But clearly the child would be safer not cut at all - and that’s not even taking into consideration the risk from hospital-acquired drug-resistant infection.
And, to expand on my objection to even the “minor” body alteration of ear-piercing, I’ve HAD my ears pierced, multiple times (6 each ear). I know exactly how painful it is. And while ‘gun’-piercing is faster and less painful than needle-piercing, it’s also known to carry more risk of infection. I wound up with recurrent infections in my cartilege piercings, so nasty that I had to take systemic antibiotics, and let the holes healed closed. As an adult I can choose to accept those risks to my health. I will not subject my children to those risks (or myself, anymore, geez). When they are old enough to choose those risks for themselves, in full understanding of all possible ramifications…they can make that choice. Their bodies; their choice. Not mine.
I realise this isn’t strictly about the concept of a hand that’s “different”, but on a meta level, one cosmetic body modification is much like another. I would not subject a child to cosmetic body modification of this sort. For something affecting life or quality of life (cleft lip or palate, fused skull bones, heart defect, digestive tract or other organ defect, club foot that will affect normal walking, etc) then you weigh the advantages against the risks, grit your teeth, and pray a lot.
I hope the OP’s baby will be just fine. After all, ultrasounds can be inaccurate.