Have a look here. Employer has an obligation to complete a risk assessment once they've been notified in writing of the woman's pregnancy. As you will see down the bottom of that link, any breach of H&S regulations with respect to pregnant women is considered automatic sex discrimination.
However, the time limits for claiming sex discrimination mean that it would be too late for you to bring a claim for that.
If you are genuinely concerned that you have been exposed to something which you have reason to believe could have harmed your baby then I'd say your first port of call should be your GP, and you could also consider giving the Health and Safety Executive a ring and chat to one of their advisers. See here for contact details.
I get the impression though that you don't genuinely think your baby might have been harmed, it's more of a principle thing, they should have done a risk assessment and they didn't, therefore something that could have harmed your baby wasn't identified, and you are more cross about what could have happened that what actually did?
Apologies if I am reading that incorrectly, and do consider investigating further if you have reason to believe that the substance you were exposed to is dangerous and could have had an effect.
But it seems to me this wasn't deliberate, it was an oversight, a mistake that was made, and by having an internal investigation they are acknowledging that they were wrong and are taking the appropriate steps to find out why that happened and to make sure it doesn't happen again. Sounds as though they are handling it pretty well tbh, not sure what else you could ask of them?