Can anyone speak to what is required of the residency programs? My understanding is that federal law requires large employers (such as hospitals) to provide unpaid maternity leave once you've worked for more than one year. Some states have additional protections.
I can try.
Federal: First, as mentioned, there are anti-discrimination laws that make it illegal to not hire someone because they are, or might get, pregnant. So if someone interviews with us and it's obvious they are pregnant, and we can "do the math" and determine that the due date would be right after starting working, we can't hold that against them. Of course, some people could rank those people lower and "find a reason" to do so.
The main law here is FMLA. Federal FMLA allows employees to take 12 weeks of UNPAID leave for a number of reasons, which would include birth of a child. There are rules that determine which employers must abide, but any hospital / GME system will be large enough to be covered. FMLA just guarantees that you get your job back after the leave. FMLA leave can be paid if you have vacation / earned time / etc, else it is unpaid. Employees are not entitled to FMLA until they have worked for 1 year at an employer -- however many employers simply treat all employees the same and allow everyone the same FMLA courtesy.
The ADA does not cover pregnancy, unless there are complications (pre-ecclampsia, etc). All employers will work with employees who, upon return, want to pump breast milk. Employers must supply a private space (NOT a bathroom) for employees to pump, and most supply a refrigerator to store, and must provide reasonable breaks for pumping.
State: Some states may have more protections. Several states have started paid FMLA (of which the rules are different for each state). It's hard to say much more, since each state is different.
Medical Disability: In most places, post-delivery is considered a medical disability. This allows women to claim disability pay post partum -- short term disability is usually full pay, or sometimes less pay but tax free (depends on how it's done). Hence, what usually happens is that women can get 6 weeks post partum paid disability (which counts towards the FMLA "clock"), and then an additional 6 weeks FMLA usually unpaid (although could use vacation, etc).
ABIM: Training time is affected. The ABIM requires that you complete 36 months of training. They allow you to miss one month per year for leave, including vacation. unused leave/vac doesn't "roll over". But, the PD can apply to shorten training by 1 month if your performance is fine (which, if it's not fine at 35 months, it won't be fine at 36 months either, so you've got bigger problems). Practically, this allows a woman to take a single maternity leave of 6 weeks -- 4 weeks paid disability leave and 2 weeks of vacation -- and still finish right on time. You could actually stretch that a bit, if programs offer 3 weeks of vac, thats 21 days and there are 31 days in a month (well, at least some months...). So she could miss 4 weeks + 10 days paid disability + 2 weeks of vacation, for 7.5 weeks. Anything more than that and you extend for the time (but can still shorten a month). So a 12 week leave usually extends by 1 month or so. My experience is that this has minimal effect on fellowship chances -- programs know ahead that you'll be a month late, and someone has to start on a non call block, so it's no big deal.
It gets more complicated than that, but that's the basics.