Where can I find online some details of the rules by which universal credit payment are worked out?

xxxxxxxx

Can you answer a couple of questions?

Is this her first child on the claim? (I assume it is but it would be nice if you would confirm). I see you saying the relevant bit for you is "if you have 3 children and 2 are already sharing."  So the fact that you singled out that particular sentence is confusing.

Does she have health conditions and is not expected to look for work, prior to having baby? i.e. she has what is known has LCW or LCWRA?

You can get the Local Housing Allowance LHA here, put your postcode in and it tells you.  Then use the LHA bedroom calculator.
Search for Local Housing Allowance rates by postcode or local authority : DirectGov - LHA Rates (voa.gov.uk)

Can you not just tell us what the exact situation is before having baby and after having baby?

TELLIT01

Could you make an appointment with Citizens Advice to try to get more clarity?

sheramber

Just to clarify, from previous posting.

The OP and ex share child care but ex claims for child on UC claim.  Ex lives with a partner.

The OP wants ex to share the amount of UC she gets due to child  claim but does not agree with ex that all she gets extra is £315.

xxxxxxxx

So either the ex or the ex's partner could already have LCW / LCWRA and therefore no new work allowance = no change to UC award.
Housing costs I already explained, but at the end of the day it is ex's housing costs not OP's so it makes no difference.

OP, you have no legitimacy to anything from ex.