Twin gestations can be easily seen by ultrasound. Get it done at about 8-9 weeks. At that time, if twins, chorionicty and amnionicity can be accurately determined. That is vital to know for the proper management of the pregnancy.
Is your mom an identical twin or fraternal? My doctor said the chances are high if your mom or grandmother have a fraternal twin. Identical doesn't affect passing twins down and your baby's father's side doesn't count either. I was scared to death of having twins my first time to and my grandmother is a fraternal twin! But they checked everywhere and I'm only carrying one.
My mom is a fraternal twins