Can someone help me?

Emma • Expecting baby #1 January 4 2019 a beautiful baby girl and can not Wait to met her❤❤

So my doctors say I'm so 18 weeks from my last period but me and my family all think I'm farther along swell due to the fact that she is so big (as well my aunt and mother have both each had 4 kids and my mother lost twins so they were closely monitored for pregnancies why they keep saying this)