Conjoined embryos, computer artwork. Conjoined, or siamese, twins are identical twins who failed to separate entirely in the womb. Identical twins result from the fertilisation of a single egg cell which then splits. If this separation is incomplete then the twins can share anything from superficial tissues, to limbs, to major organs. Conjoined twins occur in about one in every 200,000 births and the majority fail to survive. Occasionally, dependant upon the shared tissues, twins can be separated surgically.