Are There More Than Two Types of Twins?

New reproductive technologies have yielded a vast array of novel variations on the two conventional twin types—identical and fraternal—as well as some curious twin-like sibships. When a Canadian mother of twins sent me her fascinating list, I thought about whether or not these different pairings truly qualified as twins. I have reprinted the different twinship categories I received with the explanations that she provided. My comments follow each one.

• Traditional Multiples: more than one baby born in one womb, to one parent, during the same gestational period.

Segal: I concur with this definition.

• Surrogate Multiples: when siblings are born within 90 days* of each other using one or more surrogates. (*The longest recorded birth interval between twins is 87 days or nearly three months. The twins in question, Amy and Katie Jones-Elliot, are from Ireland and appear in the Guinness World Records. The Canadian mothers simply rounded up the 87-day interval to 90 days.)

Segal: “Surrogate multiples” would not qualify as twins because they were carried by two or more women with different physiological backgrounds, genetic profiles and health characteristics.

If the embryos were created by the same father and mother, the children would share the same genetic relationship as fraternal twins and full siblings. If the embryos were the products of eggs released by one woman and fertilized by different men, they would “replay” superfecundation (when twins share their mother, but not their father), which occurs naturally. Another possibility concerns embryos created by the same man and a different woman (either used immediately or frozen for later use). A circumstance in which this occurs is sperm donation, in which women become pregnant using their own egg(s) and a male donor. This procedure can produce scores of children who are genetic half-siblings. However, this form of conception does not occur naturally—there is no reverse parallel to superfecundation. I believe that the children conceived by sperm donation can be considered half-siblings, even if they have never met. That is because some divorced individuals often have children with their former and current spouses, years apart. In addition, twins reared apart from birth and reunited become part of a twinship years later.

Here is a curious question: If one man impregnated identical twin women who delivered their babies on the same day, would the children be twins? I would say that they were not twins. That is because even identical twins are not strictly biologically identical—the prenatal environments they provide to their fetuses might differ.

Using the 90-day interval as the criterion for determining twinships, it might be reasoned that related children born within a 90-day interval to different mothers are twins; however, I would disagree as I will explain below. The next question is: What is the relationship between children conceived when identical twins marry identical twins? The children conceived by each identical couple would be genetically equivalent to full siblings, as well as double first cousins. I actually came across a case in which the infants of two identical couples were born on the same day. These children would be equivalent to fraternal twins in terms of biological relatedness and date of birth. However, they should not be considered to be fraternal twins because their mothers may have experienced different pregnancy events. Moreover, the psychological consequences to the children could prove damaging if they felt forced into what they experienced as an unnatural relationship.

• Adoption Multiples: when a parent/surrogate who bears a child and the applicant also adopts another child/children who are born within 90 days of each other; or when an applicant adopts two or more children from two or more sources that were born within 90 days of each other.

Segal: I have studied 169 such pairs of “virtual twins” or VTs. These children mimic twinship due to their close age and entry into the family. Still, I would not refer to them as multiples which implies a multiple birth. Just as fraternal twins differ more, on average, from identical twins (largely due to their lesser relatedness), it would be incorrect to think of “adoption multiples” (or VTs) as twins, because of their total lack of genetic relatedness. I believe that differing in age by no more than 9 months and arriving in the home by one year of age are justifiable criteria for virtual twinship. I say that because 9 months is usually the largest age difference between classmates, and because some newborn twins remain in the hospital for months at a time, due to their medical difficulties, while a healthier co-twin goes home.

• Same-Sex Family Multiples: when a same-sex couple gives birth, individually to child/children that are born within 90 days of each other.

Segal: There are cases in which same-sex male couples have conceived twins with the help of a surrogate. I believe that the children conceived in this way are twins because they reenact the natural twinning process I mentioned above (superfecundation). The children would share the intrauterine environment and be born at the same time.

• Combined Multiples: circumstances that meet two or more of the above requirements.

Segal: It is hard to address the question of “combined multiples” without knowing the specific circumstances. Most VTs are either two adopted children or one adoptee and a biological child. I have studied some unusual virtual twins, but I do not believe that any of them can be usefully considered in any other category

Two unusual cases are worth mentioning. A woman from the Netherlands delivered twins, conceived by in vitro fertilization (IVF), from embryos created by her and her husband. Due to laboratory error, one of her eggs was fertilized by sperm from another donor. This scenario mimics superfecundation in which both children are related to the same mother, but not to the same father. The boys were raised as twins, as I believe them to be—the biological father of one of the twins (not the woman’s husband) did not care to pursue a relationship with the family. A second case is less clear cut, but raises the interesting issue of whether a shared womb in the absence of biological relatedness qualifies two babies to be twins. A Caucasian woman was accidentally implanted with one of her own embryos and an embryo created by a Black couple and delivered healthy twins. The Caucasian couple was willing to give up custody of the non-biological twin, pending DNA test results showing that the child belonged to the Black couple. Had the Caucasian couple eventually raised both infants (one their own and one that was unrelated), I would consider them to be twins. That is because they display the essential elements of twinship: nearly simultaneous embryo creation, shared prenatal environment and time of delivery. Of course, the two siblings would be completely genetically unrelated—it is theoretically possible, but practically impossible, for two children conceived by the same parents to share no common genes.

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Comment

Some parents of twins welcome parents of twin-like children into their clubs, reasoning that many of the child-rearing issues they confront are alike. Examples would be same or separate classrooms, balancing individuality and sibship, and dividing parental attention fairly. Other parents of twins feel that their clubs should be limited to conventional twins only. I believe that all mothers and fathers raising twins and near-in-age children can benefit from sharing their experiences and knowledge, and would encourage twins club members to open their doors.

This post was adapted from a more detailed version that will appear in the journal Twin Research and Human Genetics.