Can an employer in NY refuse to let a pregnant women use rollover vacation time 3mos before birth?

My wife is due to give birth in late-March. Her employer has told her that since she is pregnant, she cannot use rollover vacation that she has accrued this year for next year (only her 2016 vacation days). Her company allows employees to rollover up to 5 days to use at their discretion during the months January-March of the following year. Her employer, upon learning she was pregnant, told her that she cannot use rollover days during the 3 months before FMLA begins, thus negating her chance at using rollover vacation days. To me it seems that this is discriminatory towards a pregnant employee, as any other employee would be allowed to use their rollover vacation as they wish, and not be told by the company how to use their accrued time off. Is there any legal premise for them to withhold her use of rollover vacation days? Thank you.