How Twins Can Randomly Happen to You

twin babies

Um, can you tell them apart?

Listen, we don’t have twins in either of our families and were not using fertility drugs. So how did we end up with two little love bugs? Read on, but first let me begin with my obsession over identical vs. fraternal.
With twins, my hubby and I are OFTEN asked, “Are they identical?” And don’t laugh, but our honest answer for the longest time was: “We don’t know.”

Granted when I was pregnant, the hubby hung his hat on the fact that there were two placentas. Then one day he was talking to a Breezy Mama Go-to favorite expert Dr. Vi and found out that she and her twin, another favorite Go-to, Dr. Gillin, each had their own placenta and assumed they were fraternal. Well, both attended medical school so they took “the test” and found out they were identical!

When I asked my doctor, “How will we know?” She said when we see what they look like. Well, they looked exactly the same, the only difference being that one weighed less. Then they had the same blood type. There was nothing official to determine if they were fraternal or identical.

Now that they are five months old (can you believe it?!?), I feel they are blatantly fraternal. But I’m their mother. That being said, I often correct friends, family, the kids and even the hubby (!!!) as to who is who because they can’t tell them apart at times!

As you can imagine, I wanted to investigate further and turned to OB/GYN Dr. Bivins to answer my questions on how to know. Plus! Since we did not use fertility drugs and twins don’t run in our family, some possibilities to explain how we had a set (psst… it could happen to you)!

How can twins be identical if they have two placentas?

There are two different placental types, monochorionic (MZ) or one placenta and dichorionic (DZ) or two placentas. DZ (fraternal) twins always have dichorionic placentation. Some MZ (identical) twins may be endowed with [two] placentas (i.e., twins that separated in the first 2 days after fertilization). Although 20% to 30% of identical twins have a dichorionic placentation, most often the placentas of monozygotic twins are monochorionic. Therefore, the minority of identical twins have two separate placentas.

Just for clarification (and, well, to prove a point to my husband), you are saying that there is a 20-30% chance that my DZ twins could be identical?


Are we or aren't we?

If two babies look identical, how can you determine once and for all if they are?


The zygosity of twins is of interest to the twins, their parents, physicians who may treat the children in the future, and to scientists. An attempt should be made to establish the zygosity at birth and to register the objective findings in the medical chart. Performing this task at the birth is particularly appropriate because of the availability of the placenta, examination of which can aid materially in the process. The most efficient way to identify zygosity is as follows: Gender examination allows the classification of male-female pairs as fraternal or DZ. The twins should also have a dichorionic placenta that may be separated or fused. Next, the placenta is studied in detail, and twins with a monochorionic placenta can be set aside as being of MZ (“identical”) origin. There then remain the like-sex twins with dichorionic placental membranes whose zygosity cannot instantly be known. They must be studied genetically with zygosity DNA testing ($800).

The only way to prove it once and for all is the $800 test? Where would I go to get this done?

Yes. We use at St.Vincent Women’s Hospital Genzyme Genetics

I’ve always heard that twins run in families, but neither my husband nor myself have twins in our families. How can it be that we had our own set?

Monochorionic [a.k.a. identical] twinning does not run in families because it is a random occurrence. [In general] naturally occurring multifetal gestations are more prevalent in females of West African descent, age of greater than 30 years old, taller and heavier females and patients who have had several pregnancies.

Very interesting! Over 30 and several pregnancies… ahem…

Long before I had my own twins, I felt that twins were on the rise because of fertility drugs. After having twins without any drugs, I started thinking about all the twins I was surrounded by growing up. Are twins any more common today or is the percentage still pretty much the same before the popularity of fertility drugs?

The incidence of twinning is increasing as our population ages and a new technology—assisted reproductive technology (ART)—is becoming widely used. Not only have artificial reproductive techniques led to a marked increase in higher-order multiple births (triplets, quadruplets) but also they are followed by an increase in prematurity rates and congenital anomalies.

Can you discuss some remarkable traits that you have learned about twins?

Non–right handedness is found not only in MZ [identical] and DZ [fraternal] twins but also in their close relatives at a higher rate than would be expected in the general population.

Do you have suggestions for raising twins as individuals (vs. always as “the twins”)?

My personal opinion is to treat them as individuals. Treat them as separate people and cultivate their individual personalities.

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About Harold A. Bivins Jr., MD
Maternal Fetal Medicine Specialist
St.Vincent Women’s Hospital (Indianapolis, IN)

Dr. Bivins is a graduate of the Mercer University School of Medicine in Macon, GA. He completed his OB/GYN residency at the Medical Center of Central Georgia in Macon, and his Maternal-Fetal Medicine Fellowship at the Medical University of South Carolina in Charleston. Dr. Bivins is board certified in obstetrics, gynecology and maternal fetal medicine. Before joining St.Vincent, he had served as a maternal-fetal medicine physician at Atlantic Maternal Fetal Medicine in Morristown, NJ. Before that, he served as an assistant director of perinatology, chairman of the Department of Obstetrics and Gynecology, and director of Maternal-Fetal Medicine at Memorial Medical Center in Savannah, GA. He has also held faculty appointments at Medical University of South Carolina; Medical College of Georgia; and Mercer University School of Medicine.

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  1. Very interesting! Your girls definitely look a *lot* alike (but I can tell ’em apart…mostly!). My husband is an identical twin, although after reading your post I inquired as to how he knows for sure. Turns out they don’t. But they definitely look alike and are mirror-image (one left-handed, one right-handed; moles on the opposite sides, etc).

  2. I am a twin and the doctor told my mother we were fraternal at birth due to two placentas. During later doctor visits her doctor said we must be fraternal as we looked “identical” (same hair and eye color, same features, blood type, etc.). We’ve always told people we were identical twins and finally had a DNA test in our 40’s (our test only cost us around $200!) and we are indeed identical.

  3. Alice,

    I am also a twin and interested in having my twin and I take the test you referred to. Where can we get it?
    Thanks in advance for your help!

  4. My twins are about to turn 5…when I had my ultrasound determining sex the tech told me that they appeared to be identical or monochorionic. It appeared that I had one placenta but never did find out if it was “fused” or not. To this day I have no idea if they are identical or fraternal but they look exactly a like. Twins do run on my dads side of the family but I have been told that twins should run through the mothers side in order to be genetic/fraternal. Your article is very interesting to me because it is something that I would like to know one day.

  5. Kathy – I didn’t know your husband was a twin!
    Alice – wow! That is so interesting that you didn’t know for sure you were identical until your 40’s! I wonder if my girls will ever know -lol.
    Jo – I’m with you! I’m so curious to officially know. Although it continues to be fun to guess.

  6. Interesting reading! I have twin girls, born in 1975. No history of twins on either side of the family. They were born 2 weeks before the due date, I had absoultly no problems carrying them of giving birth to them. The 2nd to be born was breech. They were born 9 minutes apart; they weighed the same at 5lb,9oz; measured the same in head and belly circumfrence and were the same length. They were healthy and began to grow from day one! They took the same classes all thru school and colllege, ran crosscountry and played basketball. They are both Pharm reps! I had a boy before them and a boy after them. How blessed I am and the whole family.

  7. If you haven’t already gotten the DNA test, do it!! My twin sister and I used proactive twice! They were under $200, and it was worth if for us to know. We had always been told that they weren’t sure if my sister and I were identical, or fraternal. Being a twin, you are asked that ALL the time. Quite frankly people were getting sick of my placenta story (and how they didn’t check it, yada yada).
    What made us ultimately do the testing in our early 30’s, was because I found out that I was pregnant with twins. I thought for sure my sister and I would be fraternal because hyperovulation is genetic, but much to my surprise, we were identical.
    My twins had 2 placentas, the same blood type, and a 20% chance that they were identical. They looked so much alike, but I wasn’t rushing out to get them a DNA test.. until, my twin sister got pregnant and had her first ultrasound, and confirmed identical twins (shared placenta and so on).. This is what prompted us to the dna testing on my girls.. just to see if we were all identical.. And we are.
    I haven’t been able to find another set of identical twins, that have each had identical twins. And I’ve been looking!! My sister and I are mirror twins. She’s a lefty, I’m a righty, she’s nearsighted, I’m farsighted, she’s got a widow’s peak, I don’t.. she had id boys, I had id girls.. it goes on and on..
    Thanks for taking the time to read this! Best of luck to you and your twins!

  8. Deanna SEna says

    Love learning about twins. My boy girl twins will be three next Sunday! Enjoy every moment!!!!

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