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Pamela Prindle Fierro

Forum Friday: Twins of Different Sizes

By January 30, 2009

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Miarex asks, "I would really like to know ... actually, I'm dying to know ... about why sometimes the elder in the twins, I mean the one who was born first, is smaller in size than the one who was born next? You know,I've seen twins in this sort so many times, so I am wondering how did it happen? Sometimes the elder one can be shorter in height...or maybe thinner in body size....this can be seen precisely when they turn into teenagers especially...does it has to do with the contains of nutrients obtained when they were in the womb...or any other reasons??? Please help..I would really like to know...."

Miarex, there is no one specific answer to this question. There are many different reasons why twins can be different sizes. Although you may have observed the described scenario (firstborn twin smaller than second born) to be commonplace, I don't believe that, statisically, it would prove to be the standard. (I'm not aware of any recorded research on this issue; it would be interesting to see some real numbers!) In many cases, the second born twin is smaller, while in other cases, one twin is smaller at birth but outpaces their twin as they grow up. Birth order doesn't really impact the size of the babies.

However, to address your question about why any two twins would show a disparity in weight and size, there are several reasons. For fraternal (dizygotic) twins, the answer is simple genetics. Just as any two siblings may be taller or shorter or thinner or wider, fraternal twins inherit their own unique set of DNA characteristics that influence their size. Second, environmental influences may be at play. Monochorionic twins who share a placenta may be in competition for nutritional resources, and the twin at a disadvantage experiences some level of growth restriction. Twins with TTTS (Twin-to-Twin Transfusion Syndrome) will often be of different sizes. The bottom line is that for every set of twins who are different sizes, there are as many reasons to explain the disparity.

Miarex, Thanks for posting your question. I'm sure it will spur some interesting discussion! Please stop by the message forum and share your thoughts. Are your twins different sizes? Is the firstborn twin the larger one or the smaller one? Why do you think this is the case?

Share Your Thoughts
Why Are Identical Twins Different?
Birth Order and Twins: Does it Matter?
October 1, 2009 at 9:54 am
(1) Kent says:

I have a followup question… My wife and I have 1 month old twin boys. They were born 7lb9oz and 6lb13oz. They are identical and shared a placenta and so I understand the difference in birth weight.
After birth, however, I expected their difference in weight to decrease. In other words, I expected the smaller twin to catch up. Instead, there is now a difference in weight of over 1 lb.

Our pediatrician said this is normal, that they will probably stay different sizes until they are about 1 year old, but we are still a little concerned.

Is this normal? Are there others who have seen this with their identical twins?

October 29, 2009 at 2:05 am
(2) lismneina says:

Hello ,
Im new to the forum and just wanted to introduce myself, i’m Tim . I’ve been lurking around the forum for quite some time looking up info but finally decided to make an account.

October 6, 2011 at 8:09 am
(3) Jennifer says:

I have 9 1/2 month old twin girls, they were fraternal. Baby A weighed 5lbs 1 oz, Baby B weighed 5lbs 4 oz at birth. Now Baby A weighs 18lbs 2oz and Baby B weighs 21lbs 14oz. Is this normal? Should I be concerned about the difference. I am not worried about the lbs, just concerned that there may be a reason that Baby A is smaller.

September 15, 2012 at 11:25 pm
(4) ayesha says:

iam expecting non identical twins boys i am now 21 weeks pregnant..on my last detail scan dr told me that one baby size is smaller then the other n there is weight difference of 100gm..i am so worried is this normal ??they r suggesting some test but i am so scared..plz do reply ..is this size differnce is normal in nonidentical twin??

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