How soon can twins be detected?

The presence of fraternal (dizygotic) twins on an ultrasound can be detected as early as one week and four days past ovulation (approximately three and a half weeks of gestation), according to The Mayo Clinic. Identical (monozygotic) twins may not be identifiable until at least two weeks (or four weeks gestation). This is because fraternal twins result from two (or even more) eggs being fertilized, while identical twins form when just one fertilized egg splits in two early in its development.