How
Twins are Formed |
SAMBA wishes to thank Marco (father of five, including 18 month fraternal twins) for preparing the following information.
Finding Out
As we all know there is no greater joy than finding out we’re
having a baby. Right from the moment of seeing the positive pregnancy test
to the delivery. Finding out that there are two or more babies can be both
exciting and scary and sometimes a mixture of both. In 2000, 245,697 children
were born in Australia. Of that group 3,800 were twins and 102 were Higher
Order Multiples (triplets or greater). That's over 1½ percent of the
children born nationally during 2000. This is lower than the 2.7 percent in
United States of America and 1.6 percent in New Zealand.
(3101.0 - Australian Demographic Statistics, Australian Bureau of Statistics,
Jun 2001).
Although the national birth rate has declined over the last few decades, there has been a steady increase in the number of multiple births. Luke and Eberlein (1999) indicate that this rise may be caused by the average age of mothers rising, 'a woman who is 35 to 40 years old is three times more likely to give birth to fraternal twins than a woman between the ages of 20 to 25' (p.12). Tiglof (1998) agrees that age is a factor and adds fertility drugs, IVF, inherited ability (through the female bloodline), women who have already had children (the more children, the greater the chances), a woman who's already had twins and race (some races have higher twinning rates then others) have also impacted on this trend.
Monozygotic twins (commonly known as identical twins) are the result of a single egg being fertilized by a single sperm and then dividing to form two separate embryos. As identical twins share the same DNA then they are of the same sex. They are not hereditary and only occur approximately once every 250 live births.
Dizygotic twins (commonly known as fraternal twins) are the result of two separate eggs being fertilised by two different sperm, so that two embryos are formed. The embryos can be either the same or different sexes. Unlike Identical twins, fraternal twins can be hereditary on the mother’s side.
'The reason fraternal's run in families is because some women inherit the ability to multi-ovulate. Daughters of women who have had fraternal twins are almost twice as likely to have fraternal's themselves; and sisters of women who have had fraternal twins are over two-and-a -half times more likely to conceive fraternals' (Simpson & Paviour, 1994, p.197)
Despite this ability being passed through the maternal line, men have no effect on twinning 'even if he himself is a twin' (Simpson & Paviour, 1994, p.197). Fraternal twins occur approximately once every 100 live births. There has been a rise in the number since the use of fertility treatment has become more common, as some drugs encourage multi-ovulation to increase the chances of a successful pregnancy.
Conjoined Twins (commonly known as Siamese twins) are rare and occur when a single egg does not separate completely. This causes the twins to remain joined in some way. In some instances they can be separated by surgery. However in other cases the twins share common vital organs and cannot be separated without the loss of one or both twins.
Monozygotic or Dizygotic?
For some people the answer will come early and be fairly conclusive. For others
it can be difficult to determine whether their twins are identical or not.
As most parents will tell you it can be far less important to them than others.
The first question asked of most parents of twins is 'Are they identical?'
(It's even asked to parents of boy/girl twins!).
'Unless your twins are a boy and a girl, determining twin type can be tricky, especially immediately following their birth.....The only reliable method to determine twin type is DNA testing or blood testing of the umbilical cord' (Tinglof, 1998, p.5). If you follow this link to the Twin registry it will give you details of two Australian firms that provide DNA testing.
References:
(3101.0 - Australian Demographic Statistics, Australian Bureau of Statistics,
Jun 2001).
http://www.cyh.com/HealthTopics/HealthTopicDetails.aspx?p=114&np=99&id=2151#1
Luke, B & Eberlein, T (1999), 'When you're expecting twins, triplets, or quads', HarperPerennial, New York.
Simpson, L and Paviour A (1994), 'More than one: Twins and multiple and how to survive them', Simon & Schuster, NSW.
Tinglof, C.B (1998) 'Double Duty: The parents' guide to raising twins, from
pregnancy through the school years', Contemporary Books, Illinois.
