Marchmum, congratulations on your pg.
First thing's first, you do not have to tell your employer that you are pregnant until the end of the 15th week before your due date. Although obviously they will appreciate a bit more notice than that, don't feel obliged to tell them now if you don't want to.
Secondly, it doesn't matter if it looks like you were withholding it at your interview - your pregnancy is completely irrelevant to your interview for a new job, there is no need to bring it up and certainly no need to feel guilty about being pregnant or about not telling them before you were offered the role. Too often I see women feeling guilty about their pregnancy and how it will disrupt either a current or future employer. You must not feel like this. You must think about yourself first, your rights are protected by the law, and too many talented women discriminate against themselves by not applying for promotions/not taking jobs because pregnancy gets in the way.
In terms of what you should do, if you have a reasonable good relationship already with your new line manager, if you really want to have a conversation about it before you start, you could give your new manager a ring.
However I would be inclined to wait until you start, tbh. You will presumably have some sort of meeting with your manager either on your first day or shortly afterwards, I would bring it up then. Just say you are delighted to be starting work with the organisation, but would like to let him/her know that since the interview you have discovered that you are pregnant, and you thought the fairest thing would be to let them know straightaway. Reassure him/her that you are fully committed to your new role and looking forward to working with him/her and working in the new organisation.
Your new manager is likely to be disappointed of course, but I think a chat in person during which you can also be enthusiastic about your new role, and getting started, learning all about how things work, etc, will mean it's less of an issue. If you just phone or write letting them know you are pregnant, or worse, do it through the HR dept rather than with your new manager directly, it will come across worse.
You can take the initiative and alleviate any disappointment they feel with a positive discussion about taking things forward in your new job, and although of course they will still be a bit disappointed, they should be impressed by your honesty, mature way of handling the issue, and clear demonstration of commitment to them and enthusiasm about your role in the organisation. I know I would be.
And just a quick note about your rights. You will get ordinary and additional maternity leave, so you can take a year off. You won't be entitled to statutory maternity pay, however you should get Maternity Allowance, which is available to women who have spent 26 weeks out of the 66 weeks leading up to her due date in work, either at their current or a previous employer, which you will have done I imagine.
Good luck with your new job and the rest of your pregnancy, have confidence in the fact that they recruited you because you were the best for the job, not because you weren't pregnant, do a good job, demonstrate your commitment and maturity and hopefully you will have a positive future with your new organisation.