Fraternal Twins vs. Identical Twins – What’s the Difference?

By (embryologist), (gynecologist), (embryologist) and (fertility counselor).
Last Update: 05/20/2014

Multiple pregnancy is considered as a high risk type of pregnancy, since the potential risk of having a complicated pregnancy is higher. Multiple births refer to the delivery of twins and higher order multiples (e.g. triplets, quadruplets, quintuplets, etc.)

Multiple births occur very rarely; however, they are more likely to occur after undergoing a fertility treatment in which implantation of more than one embryo has occurred. Over the last few years, researchers in the field of reproductive medicine have been trying to prevent the proliferation of these types of pregnancies.

How do multiple births occur?

Multiple births occur as a consequence of different processes, which, in addition, will determine the type of twins:

  • Monozygotic or identical twins: derived from only one egg. They share the same genetic information, the same sex and almost the same physical appearance. The frequency of having monozygotic twins is about 30%.
  • Multizygotic twins: derived from fertilisation of more than one egg by different spermatozoa. It is commonly known as twins (dizygotic or non-identical twins) or triplets, quintuplets, etc. They share a physical resemblance like with any other brother or sister. They may be of different sexes. Dizygotic or non-identical twins are more common (70% of cases) than monozygotic or identical twins.

Factors favouring multiple births

There are several factors favouring multiple births:

  • Family history: previous history of multiple births on a woman's side of the family increases the chances for multiple births. Also a history of multiple births on the man's side may increase the chances for multiple births on his daughter. It is a myth that this type of gestation skips a generation, although it is not impossible.
  • Age: women over age 30-35 are more likely to develop a multiple pregnancy.
  • Assisted reproduction: as mentioned earlier, many cases of multiple births have occurred after undergoing fertility treatments. However, researchers in the field of reproductive medicine have been trying to prevent the proliferation of these types of pregnancies.
  • Taking birth control pills continuously: some studies suggest the possibility that a boomerang effect occurs after taking birth control pills.

Multiple pregnancies present a slightly different series of symptoms from those of a singleton pregnancy, such as increased nausea and vomiting. An increased uterus size is also an indicator of multiple births.

Problems associated with multiple births

Multiple births might carry the following risks to the health of the mother:

  • Premature delivery
  • Pre-eclampsia
  • Low birth-weight
  • Gestational diabetes
  • Chromosomal alterations: genetic studies are advisable in these cases.
  • Twin-to-twin transfusion syndrome (TTTS): it is a prenatal condition in which twins share unequal amounts of blood.


When pregnant with identical or non-identical twins, triplets, etc., women should bear in mind a series of recommendations to have an uncomplicated pregnancy as far as possible. Here are two recommendations:

  • Rest: unlike singleton pregnancy, women usually need extra relax.
  • Further medical evaluation: a larger number of ultrasound scans should be conducted and also the health care provider should be visited more often. Early diagnosis is important to carry a pregnancy to term and be prepared for labour -which will be probably a caesarean birth, although not necessarily.

Once the children are born, the mother should be aided by all means in order to raise her children.

FAQs from users

Is a preterm birth more likely with twins?

By Manuel Aparicio Caballero M.D., M.Sc. (gynecologist).

According to the World Health Organization (WHO), a preterm birth occurs when the child is born before the week 37 of pregnancy. Some of the most common causes of preterm birth are:

  • Overdistention of the uterus: loss of tone in the uterine musculature that prevents this organ to recover its normal size.
  • Infection or uterine inflammation: certain bacteria can damage fetal membranes by causing its rupture and triggering a preterm birth. An infection that affects the uterus directly may lead to preterm birth as well.
  • Decidual bleeding: a type of vaginal bleeding that may occur while a woman is pregnant.

There exist many risk factors that can lead to premature birth, including obesity, high blood pressure, etc. A multiple pregnancy is one of these factors.

Which specific risks or dangers are involved in a multiple pregnancy?

By Manuel Fernández M.D., Ph.D. (gynecologist).

Although the potential risks for the fetuses and the mother are becoming increasingly controlled and less frequent, it is precisely in cases of multiple births where problems for the mother may raise, such as high blood pressure, diabetes, prematurity, problems at the time of delivery, etc. As for the baby, the potential dangers are those related to prematurity, most of them considered as mild and transitional, although some may become a serious health issue.

What are the chances of twins with donor eggs?

By Rebeca Reus BSc, MSc (embryologist).

The chances of twins with egg donation are higher than in normal IVF cycles using the woman's own eggs due to the young age of donors, which translates into very healthy oocytes. This is the reason why multiple embryo transfers are unadvisable.

On average, if more than one embryo is transferred, the likelihood of becoming pregnant with twins is about 40%, while the chances of carrying triplets or greater is roughly 4%.

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Authors and contributors

 Cristina Mestre Ferrer
Cristina Mestre Ferrer
B.Sc., M.Sc.
Bachelor's Degree in Biological Sciences, Genetics & Human Reproduction from the University of Valencia (UV). Master's Degree in Biotechnology of Human Assisted Reproduction from the UV and the Valencian Infertility Institute (IVI). Embryologist at IVI Barcelona. More information about Cristina Mestre Ferrer
 Manuel Aparicio Caballero
Manuel Aparicio Caballero
M.D., M.Sc.
Bachelor's Degree in Medicine from the University of Murcia. Specialist in Obstetrics & Gynecology. Master's Degree in Human Reproduction from the King Juan Carlos University and the IVI. Currently, he is part of the team of Tahe Fertilidad (Murcia, Spain). More information about Manuel Aparicio Caballero
License: 303008030
 Rebeca Reus
Rebeca Reus
BSc, MSc
Degree in Human Biology (Biochemistry) from the Pompeu Fabra University (UPF). Official Master's Degree in Clinical Analysis Laboratory from the UPF and Master’s Degree about the Theoretical Basis and Laboratory Procedures in Assisted Reproduction from the University of Valencia (UV). More information about Rebeca Reus
Adapted into english by:
 Sandra Fernández
Sandra Fernández
B.A., M.A.
Fertility Counselor
Bachelor of Arts in Translation and Interpreting (English, Spanish, Catalan, German) from the University of Valencia (UV) and Heriot-Watt University, Riccarton Campus (Edinburgh, UK). Postgraduate Course in Legal Translation from the University of Valencia. Specialist in Medical Translation, with several years of experience in the field of Assisted Reproduction. More information about Sandra Fernández

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