As moms of twins …
… especially those whose twins resemble each other, we have become quite accustomed to being approached by curious strangers and having this oh-so familiar question asked: Are your twins identical?
Twinning occurs in approximately 32.6 in every 1,000 births (2008 statistics) and with the increasing use of fertility medications and treatments, these statistics rise slightly each year. In 1980, only 18.9 in every 1,000 births were twins (1/3 of those twins being identical, and the other 2/3 being fraternal).
For those readers who may be expecting a set of twins, and for those who are just fascinated by twin facts (like me), you’ll enjoy the following info!
When referring to the types of twins (identical or fraternal), what’s really being referred to is zygosity, defined as the characteristics of the cell union at or shortly after conception. Identical (monozygotic = one zygote) twins occur when one fertilized egg (zygote) divides into two identical parts. Identical twins possess the same genes (DNA), and may share many similar characteristics. However, since environment plays a large role in one’s physical appearance (in addition to genes), identical twins can actually look different, especially as they age and are exposed to various external factors such as nutrition and physical activity. Identical twins are always the same gender.
Fraternal (dizygotic = two zygotes) twins occur when two separate eggs are fertilized by two separate sperm. They are basically the same as any sibling would be to another, but are conceived within a day or two of the other, share their mother’s uterus at the same time, and are usually born within minutes of each other.
Science has theorized a third type called Polar Body twinning, which occurs when an unfertilized egg divides into two parts and each part is fertilized by a different sperm. The twins would then share one-half of their identical gene set (from their mother). Because it is the father’s DNA that determines gender in all pregnancies, the twins can be either same-gender or male/female.
An ultrasound may be able to determine if a mother is carrying identical or fraternal twins. Some identical twins share the same sac and/or placenta. Some identical twins, however, can have separate sacs, and the placentas of fraternal twins can fuse together to appear as one placenta. Fraternal twins do not share the same sac.
Once the babies are born, genetics testing (through a painless swabbing of the inside of the cheek) is the only real way to determine if twins are identical or not (that is, if they don’t already look completely different from each other, and also, of course, if their genders differ!)
Additional twin facts:
The only type of twins that “run in the family” are dizygotic, because a mother’s genetic tendency to hyperovulate (ovulate more than one egg in one given cycle) can be passed down to her daughter. Although there is really no proven reason know for why monozygotic twins occur, it has been speculated that a rare enzyme found in sperm may cause the fertilized zygote to spontaneously split in two separate zygotes at some point during the blastocyst stage (during which tremendous, rapid cell division occurs) as it moves from the fallopian tube and arrives in the uterus prior to implantation. Conjoined (also referred to as Siamese) twins occur when the final division never fully completes. Conjoined twins can never occur in fraternal twinning, and can only be same gender.
As for my three pairs of twins, I have all fraternal sets. I have two girls (with whom while I was pregnant were suspected to be identical); a boy/girl set; and finally two boys! Having been pregnant three times each with twins so I really don’t know what it’s like to be expecting a single baby. From having none to two, then to four, and then to six has been an amazing experience filled with many blessings as well as challenges! Looking back to when we were planning just one baby seems like a lifetime ago, but it all has been a journey I thank God my husband and I were chosen to travel.
For more comparable facts about twins types, please refer to this Zygosity Chart comparing Identical, Fraternal and Polar Body twins at multiples.about.com.