Twin dating ultrasound
Also known as fraternal twins resulting from independent fertilization of two ova, and always result in dichorionic - diamniotic pregnancies.This feature alone cannot be used to distinguish dizygotic from monozygotic pregnancies as approximately 20% of monozygotic pregnancies will also be dichorionic-diamniotic (resulting in ~80% of all twin pregnancies being dichorionic-diamniotic).
Detecting twins with ultrasound is the best confirmation that you're having two babies.
A very early pregnancy in the uterus with a vaginal scan (4-5 weeks) will only show a small black circle. At about 5-6 weeks, a tiny structure termed the fetal pole becomes visible, along with a ring-shaped structure called a yolk-sac.
At 6 weeks there should be a visible fetus, yolk-sac and the visible flicker of a fetal heartbeat.
The risk of complications and therefore the prognosis is worst for mono-mono and best for di-di: Amniotic fluid discordance is the most reliable predictor of poorer outcome.
Monoamniotic twin pregnancies share the one amniotic sac and are prone to entangled cords, as well as all the monochorionic complications (since you cannot be dichorionic-monoamniotic).