when the tenancy period has ended, the tenant probably goes onto a rolling contract whereby the landlord has to give two months notice and the tenant one month's notice.
no, the landlord doesn't have to cite a reason, but if the tenant were to find out the reason then the tenant would potentially be able to claim discrimination. (grey area).
the landlord shouldn't be re-credit checking an existing tenant, if extending their contract, because there would be no justification for doing this, unless they have been having difficulties paying their rent. in which case, surely non-renewal of tenancy would be the default position.
i have to say, i think the OP is being extraordinarily unfair of wanting to end the tenancy due to pregnancy. as others have pointed out, the OP has no knowledge of the OP's finances - a credit check will not tell you whether they have savings, financial support from family/friends, a trust fund or whatever. there is therefore no reason to suddenly want a guarantor.
personally, if i were their landlord and there were no other issues (and I wanted a longterm tenant) i can't think of a more reliable option for a tenant than one with a young family who is highly unlikely to want to move in the short-term future.
and to give you my story a little more - the landlords were absolutely fine with me being their tenant and i was a very good tenant. when i did have to move (through no fault of my own) awhile later, they gave me a good reference and a present when i left.
i had been made homeless when i was pregnant - one of the most distressing and terrifying circumstances (and I was single). i had a job and would never have had a problem paying my rent. my absolute priority was finding and keeping a roof over my head for myself and my baby.
what kind of a cold-hearted person would want to turf someone out on the streets just for being pregnant if they were doing nothing wrong?