He had spent the first few days obsessing over how he could be pregnant. He knew he had taken his potion before his heat had started. He also knew that the potion he had been testing on himself could have interfered with the contraceptive potion. It was only a very small chance but it was still a possibility. That was why Harry had agreed to cast the contraceptive charm as well. Had Harry forgotten? Severus would never know if he did or not, he was not lucid enough during his heats to know anything much. Surely Harry would not have forgotten?