Aspirin to prevent pre-eclampsia

Hi MTBs/Mummies Gynae prescribed 150mg of aspirin, was told to take 1.5 pill and discard the 0.5 pill. I’m confused on why is there a need to throw the half pill away as it’s wasted. Also, why am I prescribed 150mg instead of 100mg? I’m worried about the dosage not being right if I cut the pill, as there is no line indication on where to cut. Anyone had similar experience?