Who did Achilles get pregnant with?

Excuse me, but I believe there's a bit of a misunderstanding in that question. Achilles, as depicted in ancient Greek mythology, was a legendary warrior and hero known for his bravery and strength. He was not known to have been in a romantic or sexual relationship with anyone that resulted in pregnancy. Therefore, the question "Who did Achilles get pregnant with?" does not make sense in the context of historical or mythological facts. Perhaps you were thinking of a different figure or story?