One Placenta, Two Placentas, Identical, Fraternal?


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(Or… “How can I tell if my twins are identical or fraternal?”)

There seems to be so much confusion over types of twins. Are they identical? Are they fraternal? Many parents of multiples are given incorrect information from their ultrasound technician or doctor, who often state that since their twins had separate sacs and separate placentas they are fraternal. This is not necessarily the case. Here are the different configurations of twins in the womb and a brief description of each type of twin set.

Monozygotic Twins (abbreviated MZ) – more commonly referred to as “Identical Twins”

MZ twins are the result of one egg being fertilized by one sperm, then subsequently splitting into two separate embryos. They will be the same sex, have same hair and eye color, blood type, dental impressions, etc. but can still have many differences, resulting from a different balance of maternal and paternal genes. The term “identical” is becoming less popular as our understanding of the MZ twin type develops. These are individuals who do share an awful lot but they are not by any means identical. This probably is a big reason why so many parents of MZ twins insist that theirs could not be identical – they can simply see too many of their differences.

In the womb, they are laid out in one of the following configurations:

  • 1 placenta, 1 chorion (the outermost amniotic sac), 2 amnions (the innermost amniotic sac) – this is the most common configuration of MZ twins
  • Fused placenta (was 2 separate placentas but at some point during the pregnancy they fused together), 2 chorions, 2 amnions
  • 2 placentas, 2 chorions, 2 amnions
  • 1 placenta, 1 chorion, 1 amnion – this is the most rare (approximately 1%) and most at-risk configuration, as the babies not only share the same placenta but the same inner and outer sac

The later the fertilized egg splits, the more alike the twins will look and more likely they will share a sac and/or placenta in-utero.

Dizygotic Twins (DZ) – more commonly referred to as “Fraternal Twins”

DZ twins result from the mother releasing 2 eggs which are then fertilized by 1 sperm each. This leads to either same sex twins (about 50% overall, although boy-boy twins are half as likely to occur as girl-girl twins) or boy-girl twins (the other 50%). DZ twins may or may not look alike. They are simply siblings who happened to share a womb. They’ll share (just as any other full siblings will) somewhere between 25 and 75 percent of their genes.

In the womb, they are laid out in one of two configurations:

  • 2 separate placentas, 2 chorions, 2 amnions
  • Fused placenta (just as can occur in MZ twins), 2 chorions, 2 amnions

Sesquizygotic Twins (SZ) – also referred to as half-identical or polar twins

SZ twins occur when one egg splits prior to fertilization but each are then fertilized to become two embryos. Twins of this type would share their maternal DNA, although differences still do occur just like in any other twin type, but having each been fertilized by a separate sperm they would not share “identical” DNA from their father, hence the term half-identical. This twin type is still theoretical as there is currently no test to determine this particular zygosity type.

In the womb, they would be laid out in one of the same two configurations as DZ twins (see above).

So how can you find out which type of twins yours are?

Well, first there’s the process of elimination. If your twins are boy-girl twins then you know they are DZ. If they have a different blood type, they are DZ. If they shared a sac in-utero, they are MZ. Every other configuration would require genetic testing of some sort to know for certain. Sometimes the hospital will do a pathology test on the placenta to determine zygosity but this test is often inconclusive. Sometimes providers examine the membrane that separated the babies amniotic sacs to determine if perhaps 2 chorions had fused into one but this is a very difficult and unreliable way to tell. The most reliable way to find out is to do a simple DNA test. There are many companies out there – simply do an internet search for “twin zygosity testing” and you’ll find a lot of companies. Testing is done with a simple cheek swab that the parents do themselves at home although some companies do offer blood analysis. Tests currently seem to cost parents about $180 and results usually are returned within a week or two.

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