To expand on Sassy's point, if you get HIV at the same time you get pregnant, (i.e a double whammy - he knocks you up on the same night he infects you - god sorry that sounds awful) then chances are it would not show up in those very tests designed to protect the baby from transmission anyway as they are done pretty early on around booking in time.
There are reams of paperwork these days when you get pregnant, various things to sign, consent for blood tests and what they are testing for is included in these, as is consent for things like the triple test if you have it or are offered it, nuchal fold, all that other kind of potentially "contraversial" stuff that people may well wish to opt out of.
It would be relatively easy in the excitement of it all to idly flick through and miss it - particularly if you are not in the minset of challenging but are rather enjoying just being pregnant and having something happening that makes it all offical (forms, scans, oh and how exciting is it when you get your maternity exemption card in the post!!!)
Thing is, most people are more than happy to be tested - most are fairly confident in their status or lack thereof, and others will have a matter of fact, well, may as well know type approach.
But remember, in pregnancy particularly, question everything they "offer" you, or at least know the ramifications or potential pitfalls. One intervention can often lead to another. But it is not to say you should necessarily opt out of anything.