18 weeks pregnant ?

Hi my last Period was on the 15 Dec 2019 .After that i had intercourse with my husband on the 21st onwards .in 27 Dec i took blood test but was found negative .I tested positive on January using the pregnancy test kit.So right now the baby should be around 3 months plus . The question is a few days ago i went for scanning and the doctor said my baby is 18weeks old .How is that even possible .Can anyone please help me .This is my first pregnancy .