The UKBA will not permit any applicaiton to be made by a person presently on a visitor's visa to secure any kind of long term stay. They are very strict about that and would require the person to return home in order to make a further applicaiton from there.
In the case of pregant persons who are on visitor's visas, they are only grant further leave to remain for the pupose of giving birth where they are too far in to their pregnancy to be expected to fly home or cannot be expected to fly home so soon after having birth.
She needs to make enquiries with the maternity staff who she is seeing at the moment and ask about when is the earliest that a mother with a new born child can be expected to fly. I'm afraid that I doubt it will be much after when he visa expires, if at all.
If young mother's are not expected to fly for a considerable time following pregancy then she will need to get a letter from a doctor confirming the same and apply for further limited leave to remain before her visa expires.
Note though, that person who leave within 28 days of the expiry of their visa are not considered overstayers.
If the plan is for her to come here to settle with you, then she either:-
1. needs to divorce her husband and then apply for a fiance visa in order to come to the UK to marry you within 6 months of the visa and then make a further application for a spouses visa to settle permanently
2. Investigate her eligibility to apply for indefinate leave to remain (the Italian equivalent) and thereafter to be naturalised as an Italian citiizen on the basis of her long residence there. You have to have 5 years in the UK. If she obtains Italian citizenship then she will be able to come to the UK and exercise her treaty rights free from restrictions.
Sorry it could not be better news.
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