Twin Conception, Formation, and Development: Identical and Fraternal

Twins are one of the many mysteries, joys, and wonders of pregnancy. Although science is highly advanced and the medical community is more than equipped to deal with them, it seems as though having twins still hold so much mystery for the families of those expecting, those desiring twins, and twins themselves. Unless one makes a conscious effort to learn about twins through reading books or magazines, or even taking a class, it seems that these two bundles of happiness hold so many questions within them, their conception, formation, and development.

In order to best begin understanding twins, it’s best to begin with their conception. First and foremost, both types of twins identical and non-identical (fraternal) are formed in different ways. The former’s conception bears the most striking resemblance to that of a normal single child conception. Like single child conceptions, identical twins are conceived through the ovulation and fertilization of one egg. However, where a single child conception pregnancy divides the zygote into what will become the placenta, embryo, and amniotic sac; the identical conception will further divide the zygote into two separate and nearly identical zygotes.

Identical twins can share a placenta or each develop their own. This is done before the zygote moves down the fallopian tube and embeds itself in the endometrium. (Clegg and Woollett, 1983). If the two zygotes move down the fallopian tube separately then each will separately attach itself for growth. This creates double embryos which will develop their own placentas. In turn, the zygote split can be put off until it has already embedded itself in the endometrium. This condition yields the opposite effect of the previously discussed situation creating double embryos that share a single placenta.

Although both are twins, non-identical or fraternal twins are conceived in a surprisingly different way from both identical and single child conception. Instead of a zygote splitting to create the set of twins, these twins are brought about by the production and fertilization of two separate eggs by two separate sperm. (Clegg and Woollett, 1983). As both eggs are separate they are allowed to travel separately down to the uterus.

So which is more common? Well, clearly the non-identical twins are easier to conceive as they are derived from two separate eggs. A woman can and does often produce more than one egg during menstruation. Under these circumstances it is not hard to see how woman can then be impregnated with fraternal twins. Additionally, fraternal twins may run in the family. According to iVillage’s Parenting and Pregnacy, your female relations may cause you to carry “a gene that causes you to release more than one egg at a time, making it more likely that you will have twins.” (Plumbo, 2006) Moreover, women who’ve already given birth to one or more child and are thirty or over are more likely to produce two eggs during menstruation, and thus more likely to conceive twins. (Clegg and Woollet, 1983). Ovulation inducing drugs, like in-vitro fertilization, also cause woman to produce more eggs in hopes of woman having an easier time conceiving a child. This in turn makes candidates of either treatment more likely to become pregnant with fraternal twins. On the other hand, identical twins are the result of a rather spontaneous split in zygotes and are thus harder and less common to conceive.

Conception established, it becomes important to next determine whether the twins are identical or fraternal. Obviously with separate sex twins (one boy and one girl) the twins must be fraternal as identical are always both the same sex. However, not all same sex twins are identical. This is where it becomes a little more confusing. How exactly can we tell which same sex twins are identical and which aren’t? Well, lucky for twins everywhere, science has an answer.

In order to establish whether twins are identical or non-identical, we must look to the twins zygotsity. Monozygotic twins are identical twins, meaning twins that have developed from a single zygote as opposed to the two separate zygote discussed earlier during the conception phase. Non-identical twins are referred to as Dizygotic, the result of two zygotes. (The Society of Obstetricians and Gynaecologists of Canada, N.d.). Determining a set of twin’s zygotsity is quite simple and requires only a blood test. Monozygotic twins will go on to look similar, have the same blood type, sex, and hair color, while Dizygotic twins can be more closely compared to any other set of siblings as they are usually not very much alike.

Twin zygotsity is also said to have implications beyond that of the looks of the twins. Identical twins are more likely to be treated as the same or nearly the same person and are not always thought of as individuals. Although this is only an assumption made by others, identical twins generally share a much close bond with one another than fraternal twins. Considering what these twins share, their blood type, and any possible genetic disorders, it’s not hard to see why relating to each other is easy. However, these are not the only factors that separate the two types of twins, how they are treated, and how they are viewed by others, but as that is not the focus of this paper it shall not be discussed in further detail.

Instead, we’ll turn to the development of the twins while still within the womb of their mother. Sometimes it’s hard to imagine how exactly two babies can develop within the tummy of their mother. Whether identical or not, it’s not only a lot of the mother to bear, but a lot to comprehend.

Like any other pregnancy, twins can offer their mother comfort or discomfort due to the demands upon the woman according to growth and weight of the babies. The feeling and difficulties of the pregnancy also impact the mother based on how the babies are resting and growing within the uterus. It is this specific part of development that will be addressed.

Most identical twins share the same placenta and outer membrane (chorion) and are connected within the womb by an umbilical cord. Other developmental placements apply for both identicals and non-identicals. One form includes both babies sharing a fused placenta, yet are separated by different inner (amnion) and outer membranes. Again, the babies are connected to one another through their umbilical cord. The third form details as having the babies developing in separate placenta, amnions and chorians making the babies completely separate from one another developmentally.

Another stage in development, and the last to be discussed, the twins presentation within the womb. This means the way in which the babies show themselves and grow whether it be breech or head first. As with single child births the easiest presentation is one in which the head presents first. (Clegg and Woollett, 1983). As is commonly known, this causes the least amount of problems for both mother and babies. Generally healthy babies come from this sort of presentation and Caesarean sections are rarely needed, except in extreme and/or problematic cases.

The second most common presentation shows the babies as one head first and one breech first. (Clegg and Woollett, 1983). Few problems present themselves in this birth situation, however if one does it is usually dealing with the latter child. When giving birth to this set of twins the one with the head presenting first will usually be born first.

The third presentation is one presenting both twins breech first. (Clegg and Woollett, 1983). As breech first births are highly risky and generally problematic in the delivery room these births are rarely carried out without a Caesarian section. However, with the correct precautions taken (i.e. Caesarian section) the babies can easily be delivered safely with both being healthy at birth.

The fourth is a presentation with one twin head first and the other resting crosswise. This is the rarest form of presentation (Clegg and Woollett, 1983) and also one that can be risky. Generally the transverse twin will turn during the developmental phase and clear out the problem for the mother. However, when this doesn’t happen the baby must be delivered via Caesarian section.

No matter what type of development, formation, or conception twins experience they are sure to present a number of different joyful and quite possibly not so joyful challenges. Whether identical or fraternal, same sex or not, multiple child births are still one of the most amazing and mysterious births to present themselves to families and mothers everywhere. The key is knowledge, and the more we know about twins pre-birth the easier dealing with the pregnancy and birth will be. For further information on any of the topics discussed here, see your doctor, medical journals, web pages, or the resources listed below.


Clegg, A, & Woollett, A (1983). Twins From Conception To Five Years.New York City: Van Nostrand Reinhold Company Inc..

Plumbo, P (2006). Parenting and Pregnancy: Nine Symptoms of Twin Pregnancy. Retrieved August 4, 2006, from iVillage Web site:,,43wv,00.html

(N.d.). The Society of Obstetricians and Gynaecologists of Canada. Retrieved August 4, 2006, from Women’s Health Information: Pregnancy Web site:

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