While most Parents of Twins report to TwinParenthood that it is important to them to know if their twins are identical or fraternal, the reality is that many are misinformed during prenatal scans, according to UCL researchers in a commentary piece in BJOG[1] (a journal of Obstetrics and Gynecology).Identical or Fraternal?

Out of the 1302 families with same sex twins in the cited study who stated they had been given the information by health professionals based on the formation of the placenta as seen on the prenatal scan, 191 (14.7%) were misinformed about their zygosity.

It seems that correctly identifying if twins are identical or fraternal is trickier than some health professionals may realize.

It may be that some health care professionals actually perpetuate the common mistaken belief that all twins sharing a placenta are identical, and that twins with two placentas must be non-identical. But in fact, 25-30% of identical twins can have two placentas, according to the researchers. Separate placentas can develop in monozygotic twins when the egg splits within 2 days of fertilization.

Parents may have originally been told that their twins are non-identical (or fraternal) because of the formation of two placentas. But, if your same sex twins share eye and hair color, and are often mixed up by those around them, your twins may actually be identical.  Twin Zygosity Testing (or Twin DNA Testing) might be your best option to determine if your twins are identical or fraternal.

Zygosity is the genetic relationship between two twins. With respect to genetic similarities or dissimilarities, twins are classified as identical or fraternal.

Monozygotic = 1 egg – identical twins (when one egg is fertilized by a single sperm cell and then splits).

Dizygotic = 2 eggs – fraternal or non-identical (when two eggs are independently fertilized by two sperm cells).

[1] van Jaarsveld C, Llewellyn C, Fildes A, Fisher A, Wardle J. Are my twins identical: parents may be misinformed by prenatal scan observations. BJOG 2012;119:517–518


Zygosity Test Giveaway

Our friends at Proactive Genetics have graciously offered a Zygosity test to one randomly selected TwinParenthood reader.

05/21/2012 UPDATE: Giveaway entry is now Closed. Thanks to all who entered! Our winner is Reagan K of Arcata, CA — Congratulations, Reagan!

Proactive Genetics offers a great service to parents of twins – for a fantastic price. The genetic testing is easy – gentle swabbing to remove cheek (buccal) cells – which contain adequate DNA to perform the zygosity test. And then mail it in. Easy-Peasy.  Have questions about twin zygosity or zygosity testing? Check out thier informative Frequently Asked Questions About Twin Zygosity page.

Basic (required) entry: Please visit Proactive Genetics and find the first word on their website that begins with the same letter as your first name. Then, come back and post a comment here with the word, your first name, and tell us if you were told that your twins are identical or non-identical. Do you suspect that you might have been mis-informed?

Additional Entries:

Post a comment here for EACH entry.

  1. Basic Entry (above) is required before any extra entries will be counted.
  2. Tweet: Identical or Fraternal? 1 in 7 misinformed: http://bit.ly/JXPrWL @TwinParenthood DNA Test #Giveaway (ends 5/16) #tph #multiples #twintuesday
  3. Publicy share our Facebook giveaway announcement to your own timeline (share must be visible to “public”).
  4. Post a comment on one of our Facebook giveaway announcements.
  5. 5/14/2012 Update!  We’re adding a new way to earn an extra entry: Post a photo of your twins on our Facebook wall and tell us (with the photo) if you were told they are identical or fraternal.  (Please be sure to come back and post a comment here to earn your extra entry!)


As usual, the standard TwinParenthood giveaway rules apply — read them all here.  Giveaway is open to US and Canada. Contest Entry closes 11:59:59 pm on May 16, 2012.

05/21/2012 UPDATE: Giveaway entry is now Closed.  Thanks to all who entered! Our winner is Reagan K of Arcata, CA — Congratulations, Reagan!