Confused

Due date and anniversary doesn't add up. My boyfriend and I started dating in October 16 I found out in January I was pregnant and the baby is due in sept how is it the baby is due before our anniversary... I didn't get pregnant till December so shouldn't my boyfriend and I be together 1year and 2months before the baby?...update so after reading everyones comments and talking with a friend of mine I realized I was having massive pregnancy brain.. When you add the 3 months we were dating before the pregnancy and the 9 months it takes for the pregnancy you get 12 months which is the year I couldn't understand...