Multiple births

What you need to know about the unique developmental processes of multiples, and the care they need to ensure their optimal development.

Multiple Birth

Twins and higher order multiples have unique: conception, gestation, and birth processes; impact on the family system; developmental environments; and individuation processes. Therefore, in order to ensure their optimal development, multiples and their families need access to health care, social services, and education which respect and address their differences from single born children.

Types of Twins

Monozygotic (identical) – Formed when a single fertilized egg splits in two (or more) after conception. The resulting twins are genetically alike - same sex, same hair and eye colour, and same blood type.

Dizygotic (fraternal) – Result from two (or more) fertilized eggs. No more genetically similiar than singleton siblings.

Trizygotic (fraternal) – Result from three fertilized eggs. No more genetically alike than singleton siblings.

Quadrazygotic – Resulting from four fertilized eggs. No more genetically alike than singleton siblings.

There can be many variations of zygosity within a higher order multiple set. For example, a set of quintuplets can consist of two MZ (monozygotic) children and three TZ (trizygotic) children resulting from four fertilized eggs.

Causes of Twinning

Multiple eggs are released or there is more than one ovulation. Can happen spontaneously (without the use of Assisted Reproductive Technology).

One egg is released but splits once or more (producing Monozygotic twins, triplets, etc.) Can happen spontaneously (without the use of ART).

ART - Technologies such as donor insemination; embryo transfer; GIFT; IVF; and ZIFT put more than one fertilized egg or blastocyst in the uterus with the intention of a singleton pregnancy, and sometimes more than one implants successfully. Ovulation stimulants such as bromocriptine, clomiphene citrate, gonadotrophins, and gonadotrophin-releasing hormones can lead to the release of multiple eggs.

Some factors influencing DZ twinning - the use of ART, race, maternal height and weight, and parity.

Other factors that may affect DZ twinning rates include frequency of intercourse, menstrual history (higher in those with early menarche and short menstrual cycles, lower in those with irregular menstruation), oral contraception (varying effects), season (higher in summer months), family twin history (maternal), social class, and nutrition (conflicting reports.)

The factors responsible for MZ twinning are unknown but slightly higher than expected numbers are found in twins and triplets resulting from ovulation inducing forms of treatment for infertility, whether or not these are accompanied by IVF or GIFT. In general, there is no genetic predisposition but occasional examples of familial MZ twinning have been reported.


Monozygotic twin births occur all over the world at a constant rate of 3.5 per 1000 births. Dizygotic twin births range from 6.7/1000 births in Japan, to a high of 40/1000 births in Nigeria. The two most important factors that influence dizygotic twinning are maternal age at conception, and the use of Assisted Reproductive Technology. Due to the increasing tendency of women to delay pregnancy in our society, leading to an increased use of ART, the twinning rate is increasing rapidly.

In Canada, the explosion in the number of multiple births in the last 30 years is consistent with that found in the rest of the developed world. Since 1974, birth of twins has risen 35% (per 100,000 successful pregnancies) between 1974 – 1990. The incidence of triplets and higher order multiple births has increased over 250% between 1974 – 1990. (Reference – Multiple Births: Trends and Patterns in Canada 1974 – 1990 Health Reports – Millar WJ, Wadhera S, Nimrod C) (Source – POMBA fact sheet). In 1997 an incredible 126.67 sets of triplets were born in Canada, compared to 49 sets in 1980. The number of higher-order multiple sets is increasing the most rapidly due to the increasing use of ART (especially the use of ovulation stimulants).


  1. The rise in preterm birth rates parallels the rise in the number of multiple births. Since the great majority of higher-order multiple births are premature, and smaller babies are being saved due to the advancements in neonatology over the last 30 years, the direct health care costs of multiple birth are rising rapidly.
  2. Multiple birth children are more at risk for neonatal mortality, developmental disabilities, and severe and lifelong special needs.
  3. Families of multiple birth children face increased physical, financial, and psycho-social stresses

Further information:

Zygosity and Chorionicity


Twins (and higher order multiples) can be either monozygotic or dizygotic (or trizygotic etc. depending on the number of fetuses). Zygosity has implications for the twins after birth - whether they are “identical” (which no monozygotic twins truly are), or “fraternal”. MZ twins are often treated differently and assumed to have a closer bond than DZ twins, and can always act as organ donors, blood donors etc. for each other. Monozygotic twins are also likely to develop the same genetic disorders - if a disorder is present in one twin then it is likely the other twin will develop it as well. The ONLY time that placentation can determine zygosity is if the twins are Monochorionic (therefore the twins must be MZ). Two placentas or two chorions do not mean that the twins are dizygotic. Unless you have Monochorionic twins, DNA testing must be performed to detect zygosity.


The early detection of chorionicity is one of the most critical aspects of successfully managing a twin pregnancy. Monochorionic multiples are at high-risk for Twin to Twin Transfusion Syndrome, a life-threatening condition.

Chorionicity in twins (and especially higher-order multiple pregnancies) can be very confusing. In Dizygotic (or Trizygotic etc.) twins, the number of placentas equals the number of embryos. (MZ twins may also be in separate chorions). Monozygotic twins may have separate chorions, in which case there is little implication for the success of the pregnancy. But if they share the same chorion there is potential for serious complications such as a higher rate of fetal malformations, TTTS, and the difficulties for survival of one fetus if the other one dies. If they share the same amnion as well then the risk of complications is much greater. Within one multiple pregnancy it is possible to have any number of combinations - all of the fetuses in separate chorions, some sharing chorions as well as amnions, others sharing chorions but not amnions. Therefore it is highly recommended that thorough ultrasound examinations take place as early in the pregnancy as possible, performed by technicians trained in detecting chorionicity.

Women who are expecting multiples have a need for:

A. education regarding the prevention and symptoms of pre-term labour

B. prenatal resources and care designed to avert the pre-term birth of multiples, including:

  • diagnosis of a multiple pregnancy, ideally by the fifth month, which is communicated tactfully, with respect for the privacy of the parents;
  • nutrition counselling and dietary resources to support a weight gain of 18-27 kilos (40-60 pounds)
  • obstetrical care which follows protocols of best practice for multiple birth; and when the health of the mother or family circumstances warrant:
    • extended work leave;
    • bed rest support; and
    • child care for siblings.

View other Pregnancy & Childbirth Topics