introduction of weekend work - is it legal?

[cantbelieveImquittingcoffee]

I manage a busy "leisure" centre (not exactly, but best way to explain it without giving too much personal info) which has 3 sites and a head office. When I was hired my contract states that my working hours are 5 days/shifts between 5.30am and 10pm (ie the centre opening hours) without stipulation as to total working hours of days worked. However after I started I was told that I had to work one evening, one early and one weekend shift a month but I refused on he grounds that this was never discussed during many months of negotiations about the role (which was a promotion for me as I have worked for the company previously).
I'm aware that the other managers who were hired after me had this properly agreed, so they do already work weekends etc, but I don't.
We now have a new manager and have been told today that as of 1st feb we all have to work this new pattern. I am almost 6 months pregnant so not keen to start now, plus DP is absolutely adamant that they can't suddenly make me.
Any thoughts? Is this legal? I feel like I can argue the point on the basis of my pregnancy but what about afterwards (assuming I come back to work) - isn't this changing the terms of my employment without consultation?
Sorry for the essay but any thoughts gratefully appreciated!

When did you start?

Oh yes I mean to say that! Started current role in February 2014 and haven't had to work the unsocial hours during this whole time.

I'm not clear, if the centre is open from 5.30 a.m. to 10 p.m. and you are expected to work 5 days/shifts per week, how did you ever expect that this was not going to include weekend and unsociable hours work? If you are in a role which is required in order to operate the centre (rather than perhaps an office based role) surely it would have seemed so obvious that you would have to work different shifts that it wouldn't even have been a point of discussion? Or am I missing something here?

Sorry I missed out some crucial points on my first post! The agreement of my normal working hours was Monday-Friday 9.30-6. I successfully managed to argue that as the requirement to work weekend shifts had never been mentioned to me in any of the negotiations that I didn't have to do it.
But now 2 years later I have been told that I have to, with 3 weeks notice.
So the question is, can I argue it? Does it hold up legally?

worksmart.org.uk/work-rights/pay-and-contracts/contract-changes/can-my-employer-change-my-contract-employment

Yes, they can change your contract. They must consult with you first for your views, but they don't have to take them into account and they can then give you 30 days notice of the change.

If you continue coming to work after the 30 days are up you are deemed to have accepted the new terms and can be held to them. If you don't continue to come to work then you will be considered to have breached your contract anyway.

Sorry, but unless you have a really good reason (and not wanting to won't cut it) why you shouldn't work weekends then they can make this variation

They should have consulted with you though. And you should have 30 days notice in writing of the change. I'm sure when you bring this to their attention they will make sure they do both.

The short answer is yes. As long as they have given you notice and the opportunity to discuss it (I.e.some sort of consultation, time for you to offer alternative arrangements).
If you had more service, you could in theory resign and claim constructive dismissal because the changes to your working hours are a breach of your contract.
However you need two years service to do this, so this is not an option for you.

If you think the decision to change your contract is due to your pregnancy, then you may have a claim for maternity related discrimination I.e. But for your pregnancy your employer would not have tried to change your contracted hours.
Discrimination claims are notoriously difficult to prove. Your employer would likely argue that the change was to amend an error in your initial contract and to support the needs of the business.

Thanks so much to those who took the time respond!
The link is really helpful as it shows that my employer hasn't followed the correct protocol with this, but I'm a bit distressed to realise that it is legal
It's not discrimination on the grounds of my pregnancy as it's been told to me and 2 (male) managers, but I am planning to try and refuse it on the grounds of my circumstances; while I'm pregnant there is no way I can work 6 days in a row (even if I then take 2 days off and work 4 days the next week) - I'm so exhausted by Friday there's no way I'd make it through another day too! And then once I have a baby (assuming I return to the same role) it would create childcare issues so I'm hoping if I raise both issues, on top of the fact I had an agreement 2 years ago (almost) that I don't have to do it that I will win. Let's see...

Your childcare issues are not their concern and have no bearing whatsoever.

As long as they have done a pregnancy risk assessment that doesn't forbid working six days then they can go ahead. You could ask nicely that they delay the change until after maternity leave but they are under no obligation too.

It doesn't matter re the previous verbal agreement, needs of the business can change and as long as they give thirty days notice they can vary.

If you don't want to work unsocial hours and weekends, perhaps this isn't the job for you?