Ay, first time mom worries talaga! A 10cm baby at 12 weeks sounds quite big actually, normally around that time baby is around 5-6cm lang based sa ultrasound measurements ng maraming friends ko, so definitely better to check with your OB to make sure lahat ay okay and walang dating issue sa dates or anything like that. Buti na lang may ultrasound na kayo para makita exactly kung ano ang situation, so mag-follow up lang with your doctor para sa peace of mind mo, alam mo naman how pregnancy can be confusing especially first time!