Although going into single practice is indeed less common these days, there are still several mds out there that prefer the "partner-less" lifestyle. Contrary to the previous post, you wouldn't necessarily have to be on call every night if you joined a call group (a group comprised of either individual mds or groups of mds that cover for each other during call nights). As such, you may benefit from having your own practice (without the hassle of having partners) and still have decent call. Going along the same example as above, if you joined a call group of 10, you could potentially be taking call 1 in every 10 nights/weekends.
Infertility is a great and interesting specialty that is quickly advancing in technology over the past several years. Reimbursement is great, but getting there is tough (REI fellowships are competitive and the REI boards have the highest failure rate of all the Gyn subspecialties).