Universal credit

I will be getting 90% of my pay for first six weeks but after that I think I can get help from universal credit to top up my income due to it falling below the threshold as my partner is currently doing 30 hours and probably won’t do more due to childcare being so expensive so that I can go back to work full time as I earn more. I was wondering if anyone knows when/how I claim (do I wait until I’m on SMP) and if the fact we stay with his mum would be taken into account (her earnings are only a pension and we give her money towards rent). I’m hoping we are offered our own house by then but we have said no to one and haven’t had another offer and it’s been a month and I’m due in 6ish weeks. Never had to claim anything before so I’m struggling to understand it but will need help as I’m losing about 50% of my income.