Pregnancy and childbirth involves unavoidable and irreversible aesthetic damage to the mother's body. Some people argue this is not "damage", but "change", and that it's "natural", and therefore ought to be appreciated.
I believe there's nothing about "natural" that necessarily implies "desirable", and that these changes to the mother's body are about as equally desirable as disease and death.
In addition, childbirth involves a good chance of functional damage:
Fifteen percent of women become incontinent, to some degree, of stool or
urine after normal delivery, this number rising considerably after these
women reach menopause. Vaginal birth injury is a necessary, but not
sufficient, cause of all non hysterectomy related prolapse in later life.
You can avoid functional damage by opting for a Caesarean Section, and you can mitigate some of the aesthetic impact through plastic surgery. But this is risky, and is inferior to avoiding the damage altogether. Abdominoplasty has a 5% complication rate, including a chance of lethal blood clots. Even in the optimal outcome, results involve large scars. And bolted-on boobs usually don't look right.
Depending on how you looked before, naked, you may look better after all these procedures. But if you looked great before, you're not likely to look as good. Not to mention the risks of the surgery going wrong.
Many women suffer during pregnancy, experiencing nausea, depression, or an inability to take prescription medication which they need for normal functioning. Some women, however, have few problems, or no problems at all. Their pregnancies are easy, and they experience no complications during labor.
Currently, most jurisdictions require a doctor to certify a "medical necessity" in order to use a surrogate mother.
But wouldn't it make sense for women who have already given birth, and have not experienced problems, to be surrogates for those who haven't?
The risks of functional damage are lower for someone who has already given birth, than they are for someone who has not. The aesthetic impact is also lower for someone who has already accepted changes related to pregnancy in previous child births.
Surrogates are compensated, of course. In the US, they receive between USD 20,000 and USD 50,000 for each pregnancy.
So why require a medical necessity, when both parties are willing, and it makes sense to assign the pregnancy to someone who has already done it?