What's the difference between fraternal and identical twins? And can you tell whether twins are fraternal or identical before they're born?
Fraternal twins come from the fertilization of two different eggs, whether it happens in the lab, as with in vitro fertilization, or spontaneously during a double ovulation (one in 80 to 90 pregnancies).
In cases involving the Assisted Reproductive Technique (ART), fraternal eggs are fertilized from four to eight at a time, and anywhere from three to six are presented to the uterus (by insemination) for implantation. Of these, only some "take," the others miscarrying (silently because it's so early). Twins and triplets are becoming more frequent today because of ART. The less favorable fertilized eggs are not used, which may bring up ethical questions for the prospective parents.
Identical twins (and triplets!) come from the splitting of only one fertilized egg. These twins are called identical because, of course, they will have identical DNA (genes). Fraternal twins can look similar, but identical twins usually look very much alike.
An ultrasound can tell if the twins are identical only when there's one sac. But identical twins can also be seen with two sacs and with one or two placentas. It really depends on how early the fertilized egg was split. The earlier, the more likely a doubling of everything, including placenta and sac. The later, the more likely the two babies will share a placenta or a sac.
But fraternal twins are two babies from two ovulations and two fertilizations—there must be two sacs and two placentas. So in summary, a set of identical twins may present with only one sac, or with doubling of everything; fraternal twins must have a doubling of everything.
A single-sac pregnancy of identical twins, by the way, is extremely dangerous. There's a strong likelihood of cord entanglement and fetal death. This is an extremely high-risk type of twin pregnancy. We obstetricians are always relieved to see a membrane between two sacs.