Pregnancy 16 week test - HIV?

Why are pregnant women tested at 16 weeks for HIV when the window period for HIV is 6 months? Do most cases show up within the 16 weeks? I am slightly worried as I have just found out that there is a risk that the sperm I used to conceive my daughter could have been contaminated and am extremely worried. My daughter is a big healthy baby though but I have just finished an 11 month stint of breast feeding. Serious answers please.

i thought i should add that i did have the 16 week HIV test and it was clear.

Women who are pregnant have the P24 Antigen test done, this detects for the virus itself rather than the test that detects the antibodies

The window period for the antibody test is 3-6 months.

The window period for the P24 Antigen test detects the virus itself so it can be done long before 6 months(usually 12-16 weeks) thus the reason they use it for pregnant woman. The reason for the window period of the antibody test is because the immune system can take upto 6 months to create antibodies to try and counter the virus)

The P24 antigen test is done because if they can detect hiv in the women early they can start her on medication to reduce the risk of the child getting hiv.

Maybe because if the doctors can know your HIV status they can plan for the birth. With a c-section and proper warning they can pretty much guarantee the child will not contract HIV. It's the actual tearing of birth that passed it in the past. The fetus does not contract HIV during development. We don't know why, but we know it to be true.