A single placenta and single amniotic sac with two embryos, indicate identical twins. Two amniotic sacs with an embryo in each indicates fraternal.
And just so everyone knows so you can argue with your Lib friends, babies are fully formed by 12 weeks gestation. From there on out, they simply grow. And there is a detectable heartbeat at 18 days gestation. Use those facts the next time your Lib friends refer to a baby in the womb as ‘just a clump of cells’.
Non quite there are modi and momo identical twins. Modi is common, momo is very high risk sadly