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Different Types of Twins

I am a mother who bore twins and learned many things about them.

This is a 4D ultrasound of fraternal twins at 17 weeks gestational age.

This is a 4D ultrasound of fraternal twins at 17 weeks gestational age.

Rise of Twin Pregnancy

Twin pregnancies are on the rise. This is partly due to the increased amount of women undergoing fertility treatments, and to the fact that many women are waiting until their late 30's or older to have children. According to a report from the National Center for Health Statistics, part of the Centers for Disease Control and Prevention, twin pregnancy's have risen 76% between 1980 and 2009.

All twin pregnancies are considered "high risk" pregnancies. Twin pregnancies are very stressful and demanding on a woman's body, especially an older woman's body. Twin pregnancy symptoms are much more pronounced and twin pregnancies are much more apt to have complications such as, hospital bed-rest, preeclampsia, HEELP syndrome (Hemolytic anemia, Elevated Liver enzymes, and a Low Platelet count), and PIH (Pregnancy Induced Hypertension) , just to name a few. They are also much more likely to have to have an early delivery, which may result in the preemie twins having to stay awhile in the NICU (Neonatal Intensive Care Unit). A mother with a twin pregnancy may be more likely to need to deliver her babies with a cesarean section, as one baby may be breech and the other head down. Even if both babies are head down, after the first is delivered, the other may flip, causing the need for a C-section. So, some twin births are delivered both vaginally and cesarean section. Mothers of twins also require more recovery time after giving birth.

Here are the two types of twins that will be covered in this article:

  • Fraternal twins (the most common twin type, especially boy/girl but can also be boy/boy or girl/girl)
  • Identical twins (the rarer twin type, but always either boy/boy or girl/girl)

Fraternal Twins

Fraternal twins result from the mother hyper-ovulating, releasing more than one egg during her menstruation cycle. A woman can inherit a gene, from either of her parents, which causes her to hyper-ovulate. Fertility treatments can also cause hyper-ovulation. When two eggs become fertilized by two individual sperm cells, fraternal twins result. Each twin has its own amniotic sac and its own placenta. When the two fertilized eggs implant close to one another, their placentas are so close, they fuse together, appearing to be one placenta. The placentas fusing together into one, when fraternal twins are of the same sex, can cause them to be mistaken as identical twins.

Many siblings, who are not twins, greatly resemble one another; however, some siblings don't look very similar to each other at all. The same goes for fraternal twins. In fact, they may look so much alike that some people may even mistake them for being identical twins (Mary Kate and Ashley Olsen are said to actually be fraternal twins) or they may look so different that people question if they are siblings at all. Fraternal twins are basically siblings in the womb at the same time. Other than sharing the same pregnancy together, fraternal twins are no different than any other siblings.

Types of Fraternal Twins

  • Sororal Twins - Fraternal twins that are girl/girl twins are referred to as 'sororal twins and are the rarest form of fraternal twins.
  • Superfetation- This is when twins are conceived at different times due to the mother releasing an egg that gets fertilized while she is already pregnant.
  • Heteropaternal Superfecundation-Disturbing but true- when a woman releases more than one egg during ovulation and two become fertilized by two different men, resulting in twins who have different fathers
  • Tetragametic Chimerism- This occurs when one zygote of a set of fraternal twins absorbs the other zygote and combines it into one. It will now be one person with two separate sets of DNA.
Here is a pair of fraternal newborn twins sleeping.

Here is a pair of fraternal newborn twins sleeping.

Identical Twins

Identical twins are formed from one egg that has been fertilized by one sperm and then, spontaneously, splits in half, forming two eggs. They share the exact same DNA with each other. Identical twins may or may not share a placenta and amniotic sac depending on when the egg splits. If the egg splits within the first two days, they will each have their own placenta and their own amniotic sacs. If the egg splits between three to eight days, it most likely will result in two separate sacs but sharing of the same placenta. If the egg splits thirteen days after conception, they will share the same placenta and amniotic sac. When identical twins share both a placenta and amniotic sac, there is a higher risk of complications and mortality. Identical twins that do share a placenta can develop what is called, twin to twin transfusion syndrome. Twin to twin transfusion syndrome is where blood from one twin (the donor twin) is passed to the other twin (the recipient twin). The recipient twin is large and has a large number of red blood cells in his/her circulatory system. The donor twin is small and sickly. This condition endangers the lives of both twins. If the egg doesn't split until even later, conjoined, or Siamese, twins result.

Types of Identical Twins

  • Mirror Image Twins: Only identical twins can be mirror-image twins. This occurs when the egg takes longer than usual to split, usually a week or longer after conception.The original right half of the egg becomes one individual and the original left half becomes the other. These twins are mirror images of each other. If one has a mole on their left cheek, the other will have a mole on their right cheek. If one's hair parts on the right, the other's parts on the left. In rare cases, one twin will have all their internal organs on the opposite side of the body.
  • Polar Body Twins: Polar body twins are very uncommon. They are half-identical twins. This happens before fertilization. The egg divides unevenly. The smaller half (known as the polar body) normally would dissolve and be reabsorbed into the mothers body. However, in polar body twinning, instead of dissolving or dying off, it remains and acts as a second egg. Then there are two identical eggs and they are fertilized by two separate sperm from the father. Therefore, the DNA from the eggs is identical and the DNA from the separate sperms is not.
  • Conjoined Twins: Conjoined twins, or Siamese twins, occur when the egg splits very late and not completely.

Twins Born with Different Dads

Twin Pregnancy Facts

  • If you have a family history of twins (fraternal), your chances of conceiving twins are higher.
  • Women aged 35 and up have a better chance of conceiving twins than younger women.
  • Women that consume high amounts of dairy are said to have more of a chance of getting pregnant with twins.
  • If you are breastfeeding and become pregnant, your chances of a twin pregnancy are higher.
  • Chances of having twins increases with each pregnancy.
  • African women have the highest odds of conceiving twins, followed by European women. Hispanics and Asian women have the lowest odds.

Surprising Facts About Twins

This content is accurate and true to the best of the author’s knowledge and does not substitute for diagnosis, prognosis, treatment, prescription, and/or dietary advice from a licensed health professional. Drugs, supplements, and natural remedies may have dangerous side effects. If pregnant or nursing, consult with a qualified provider on an individual basis. Seek immediate help if you are experiencing a medical emergency.

© 2014 liataylor


liataylor (author) on March 29, 2017:

Thank you for commenting, Catherine. Superfetation is common in many animals, but an extremely rare occurrence in humans. I believe there are only 11 cases documented. The last was in 2015. Conception can be days to weeks apart. The biggest age difference so far has been two months. I think they've all been delivered at the same time, like twins. Although I guess, being different ages, they aren't technically twins.

Catherine Giordano from Orlando Florida on March 29, 2017:

An excellent article about twins. I learned a lot about twins. In superfetation is the second twin conceived within days of the first month or is it a month or two later. If there is a gap of a month or mor are they both born at the same time?