So, if I am to understand you, you had a needle stick, correct? Unless you inject yourself with the needle, it is not a risk. This may be why they did not offer you PEP. With that being said, you have tested negative (expected). 4th generation tests which it sounds like you had are conclusive at 28 days and EVERY other test is conclusive at 3 months. You had a 4th generation negative and no risk if it was a simple needle stick for HIV, I'd consider it fact that you don't have HIV. Congrats on the pregnancy.
You have tested negative and your result is definitive. Practically, there is no factor that can lead to delay in sero-conversion albeit chemotherapy can but that doesn't seem to be your problem. The window period for a IV generation HIV test is 6 weeks not 12 weeks.
HIV sero-conversion rate is estimated at 0.3% after an accidental exposure to 'HIV infected blood'. I would also like to point out that you are only assuming this other person to be HIV positive.Your risk was practically non existent at the first place.
It's time for you to forget about this event and move on. No further testing is necessary.