By Cristina Mestre Ferrer BSc, MSc (embryologist).
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.

Further medical evaluation

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:

  • Heredity: 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.

Authors and contributors

 Cristina Mestre Ferrer
BSc, MSc
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