Seeing Double: An introduction to twinning

Author:  Lee Jean
Institution:  Integrative Biology
Date:  September 2005

The idea of having a twin can develop into a fascinating daydream. The fancy of having "someone just like me" exist in this universe is a source of mystery and enjoyment. For some, human development clearly has rendered this dream into reality. Identical twins are a real phenomenon, and in some areas of the world, such as Nigeria, quite common. Twins are our family members, friends, roommates, and stars on the screen. And now more than ever, with the help of fertility drugs, human reproduction has brought into being the mystery and magic of twins, triplets, quadruplets, sextuplets, and even more! But how do twins come into being in the first place, and how do they develop inside the mother's body?

Human females undergo ovulation at approximately day 14 of the typical 28-day menstrual cycle. Unlike most mammals, female humans are biologically unique in that they are cryptic ovulators. Women do not show visible signs that they are in the process of ovulation, although one generally can predict when ovulation occurs by monitoring a woman's cycle. Usually one egg is released during this time; however, some mothers may be genetically prone to multiple ovulations. In fact, members in the same family line exhibit a tendency for twinning. Although bearing identical twins tends to happen only once to a mother, having fraternal twins more than once may be likely. This suggests the existence of a strong genetic component between humans and the probability of twinning.

Without the aid of fertility drugs, pregnancies resulting in twins occur at an estimated frequency of one percent. But with the aid of fertility treatments such as gonadotropins, which can stimulate multiple ovulations, doctors can increase the frequency of twinning or further multiplicities to ten or even forty percent higher than the normal rate.

Two types of twins exist, dizygotic (DZ, or fraternal twins), and monozygotic (MZ, or identical twins). Whereas dizygotic twins are as dissimilar (genetically) as usual siblings and are often different sexes, monozygotic twins are genetically identical and therefore are the same sex.

In DZ twinning, ovulation results in two separate eggs being released from the same menstrual cycle. Incoming sperm individually fertilize both eggs. Not only do these eggs differ genetically, but the resulting individuals are usually just as dissimilar as siblings born from separate pregnancies. Since two distinct sperm fertilize each available egg, the twins are independent in terms of sexual genetic makeup as well. Only a single genetic difference in a chromosomal test, for example, will establish dizygosity. Upon completion of fertilization, dizygotic embryos implant in the uterine wall and each develop three separate fetal membranes---the amniotic sac, chorion, and placenta; just like two peas in different pods.

Compared to DZ twins, MZ twins (or identical twins, which are about half as common as dizygotic twins), face quite a different start to life. Monozygotic twin formation occurs when a sperm fertilizes a single ovulated egg; the fertilized egg then divides into two separate inner cell masses during the first 2 weeks of embryonic development, or blastocyst stage. Each cell mass develops into an embryo. Another possibility is the blastocyst splitting slightly later around this time, resulting in two embryos. Monozygotic twins must be the same sex and identical genetic makeup, since only one sperm and one egg were involved in fertilization. The three fetal membranes are usually shared, like two peas in one pod. However, if the MZ twins were formed due to a slightly later splitting of the zygote cell mass, the chorion and placenta may be shared, but not the amnion. One amnion for each twin would develop. Thus, it is possible to tell when the original zygote splits by studying which of the three fetal membranes are shared.

The processes underlying MZ and DZ twinning seem simple enough, yet what happens when things don't go quite as planned? Often, twinning may cause pregnancy complications. The womb needs to nourish two or even more individual fetuses growing rapidly. If one twin fails to survive, the other twin is seriously at risk. An embolism, or circulatory blockage, from the tissue of the non-living twin may cause the living twin to suffer and eventually die. Even if both twins survive in the womb, pre-term labor can also occur, resulting in premature infants who may have extensive physical as well as mental hardships.

In addition to the struggle for nutrients and survival, twins may be physically conjoined, or bridged together. Conjoined twins - also known as Siamese twins, after the famous conjoined siblings, Chang and Eng - may result if the inner cell mass during a monozygotic process does not separate completely. The extent of joined tissues between the twins can be as slight as shared superficial skin, or as massive as deep body and intestinal fusions. Surgery can usually correct the conjoined state, depending upon the severity of tissue integration. Even without surgical correction, Siamese twins may lead relatively normal lives. Chang and Eng were born in Siam, Thailand in 1811, with a lower thoracic ligament connecting their bodies. Yet after spending most of their lives as a fascinating and acclaimed exhibition, the twins settled in North Carolina, fathered twenty-one children, and lived to the age of sixty-two.

The study of twins continues to be a fascinating and invaluable realm of biological and genetic research. Twins separated at birth and twins raised together are studied in order to distinguish between genetically-inherited behaviors and behaviors that result from environmental factors. Sometimes tests yield eerie results and similarities too striking to be called coincidence. For example, some twins adopted separately and who live apart for years develop nearly identical habits (e.g. both twins chain smoke, give their children the same names, drive the same kind of car, compulsorily flush the toilet before using it, like a certain school subject, marry women with the same names, etc.). The mysterious combination of both genetic and behavioral traits, not to mention the quirk of fate, make twins a "one of a kind" marvel.

Suggested Reading

Carlson, B.M. PhD, MD. The Human Embryo and Developmental Biology. 1994. Mosby-Year Book, Inc. Missouri. Pages. 39-44.

Larse, W. J. PhD. Human Embryology. 1993. Churchill Livingstone. Pages. 445-446.

Mange, E. J., Mange A.P. Basic Human Genetics. 2nd Edition. Sinauer Associates, Inc. Publishers. Sunderland, Massachusetts.

Wallace, Irving and Wallace, Amy. The Two: A Biography. 1978. Simon and Schuster. Publishers. New York.

Zietman, A. and M. Brookes. Clinical Embryology: A Color Atlas and Text. 1998. CRC Press, LLC. Pages 52-65, 244-318.