Hi my partner had her first pregnancy scan yesterdayand we were informed she was 13wks and 2 day pregnant which would make conception around 20 dec but we hadn't had intercoarse until 3rd jan can you give us advise

Ultrasounds aren't always 100% accurate. Was your partner with anyone else before you? If you have any doubts or concerns, the only way to confirm the child is yours will be a DNA test when the baby is born