But with twins, there can be a variety of combinations. There may be one or two placentas, one or two amniotic sacs, and one or two chorions.

Because fraternal, or dizygotic, twins are 2 separate fertilized eggs, they usually develop 2 separate amniotic sacs, placentas, and supporting structures. Identical, or monozygotic, twins may or may not share the same amniotic sac, depending on how early the single fertilized egg divides into 2.

Monochorionic twins are identical twins who share one placenta. This occurs in approximately 70 percent of pregnancies with identical twins.

1 There can be more than one placenta (one per fetus) or a single placenta that is shared by the fetuses. The number of placentas can be an indicator of the zygosity of the twins (a term that refers to whether they developed from the same egg or from different eggs).

A twin pregnancy with two placentas and two amniotic sacs is the optimal twin pregnancy, as each baby has its own nutritional source and protective membrane. One placenta and two amniotic sacs. In pregnancies with one placenta and two amniotic sacs, you will definitely have identical twins.

Sharing a placenta or inner sac: why it’s important to know Sharing a placenta means that twins share a blood supply during pregnancy. Sometimes the blood supply is shared unequally, which can cause health problems for both twins.

It’s possible for more than one placenta to form – for example, if you are pregnant with twins or triplets. This is the most common reason to have two placentas in pregnancy. Multiple pregnancies are becoming more common due to the increase in fertility treatments and because women are having babies later in life.

Various types of chorionicity and amniosity (how the baby’s sac looks) in monozygotic (one egg/identical) twins as a result of when the fertilized egg divides. Monochorionic twins are monozygotic (identical) twins that share the same placenta. If the placenta is shared by more than two twins (see multiple birth), these are monochorionic multiples.

A dichorionic diamniotic (DCDA) twin pregnancy is a type of twin pregnancy where each twin has its own chorionic and amniotic sacs. This type occurs most commonly with dizygotic twins, but may also occur with monozygotic twin pregnancies. This type of pregnancy may have characteristic findings on ultrasound.

Monochorionic-diamniotic twins are twins that share a placenta but have their own amniotic sac. They are almost always monozygotic, meaning that they come from one egg that is fertilized by one sperm, and therefore are identical.

Dizygotic or fraternal twin pregnancies almost always have two placentas. Also called “sibling” twins or “false” twins, the babies are just as unique as any other siblings. Rarely, there have been cases of monochorionic placentation -in which twins share one placenta but have their own amniotic sac ( monochorionic-diamniotic or Mo-Di for short).