Have to disagree there witchandchips, 'length of service' has to include maternity leave.
If a company puts a clause into the terms and conditions of it's maternity policy saying that receiving enhanced maternity pay depends on having been back at work for a certain period of time, I suppose they could do that, although I've never come across it myself I have to say.
But if a maternity policy just says you must have 'xxx years' service' to qualify for something, service on maternity leave must count.
In terms of changing maternity policy more generally, I would say that just because something isn't spelled out as 'contractual' doesn't necessarily mean it isn't. Which I think america's employers seem to have just realised!
Something can be discretionary, if it says so, and if in practice it isn't just paid to everyone. But if an employer has an enhanced package that they pay to everyone automatically, or to everyone with x years' service automatically, they can't just take it away without consultation as I would say the argument would be that it has then become part of the terms and conditions of employment.
Having said that, performing a consultation exercise for reducing maternity benefits wouldn't exactly be tricky, and assuming there was a business reason for the change, there's not an awful lot anyone could do about it anyway.
In practice as an employer you'd probably change it for pregnancies going forward and make exceptions for people going off for second/third babies as america's employer is doing, and botbot's has done previously.
Botbot it would definitely be worth making fuss a second time. You may not get what you want but it's definitely worth a go if you get pregnant again. The way they phrased it last time 'exception' and all that, rather than confirming that the old policy still applied to you, sounds as though they might not budge again though.