Twice as nice - or double the trouble? Sarah Tennant
explores the fascinating facts behind the conception of
It's a rare woman who doesn't stop and smile at the sight of twins
napping in a pram or toddling about in matching dresses. From
Shakespearean comedies to Mary-Kate and Ashley, Romulus and Remus,
and the Bobbsey Twins, there's just something about twins which
fascinates people. While having dealt with a singleton pregnancy
and newborn phase has made me a devout convert to the "one at a
time" school of thought, I still find myself
drawn in wonder to twin-related pictures and
discussions on the internet.
Yet despite the fact that twins are more common
and arguably more high-profle than ever, the nitty gritty of twin
conception remains a closed book to the public at large. It was
only by accident that, while surfng online, I discovered that there
is more to twins than the popular question, "Are they identical?"
In fact, twin conception is a fascinating, complicated, and
sometimes downright wacky
Why twinning happens
Leaving aside some very unusual circumstances, twinning happens in
one of two ways. Either the woman releases two eggs during her
cycle rather than one (a process called hyperovulation) and each
egg is fertilised by a different sperm, or the woman
releases one egg which is fertilised in the normal way and
subsequently splits in two. Twins in the former case are known as
dizygotic or fraternal twins; in the latter, as monozygotic or
Thus far, conceiving monozygotic
(one-egg) twins is believed to be completely random, with no
genetic component. The worldwide rate is constant at four births
Hyperovulation, on the other hand, has
been linked to a number of factors. It is genetic, meaning that
fraternal (not identical) twins may run in
Hyperovulation often occurs when the menstrual cycle is "gearing
up" or "running down", which means that women close to menopause
and women who are breastfeeding are more likely to conceive twins.
Similarly, coming off birth control or using it erratically may
trigger hyperovulation. Environmental causes, particularly related
to hormones, have also been discovered. The highest birth rate of
twins in the world occurs in the Yoruba tribe in West Africa, and
has been attributed to a chemical in the tribe's staple food -
Animal products may also contribute,
possibly due to the hormones consumed - vegan women
have very low twin conception rates, and dairy, in particular, has
been implicated in hyperovulation. Fraternal twins are also more
common for women who are taller than average, heavier than average,
or who have had several previous children. And, of course,
hyperovulation is often deliberately caused by stimulating the
ovaries with fertility drugs such as clomiphene.
Types of twins
Two-egg twins are called "dizygotic", or less commonly "biovular";
the colloquial name "fraternal" is fairly accurate, as it
emphasises the fact that such twins are simply siblings who happen
to be conceived around the same time. Dizygotic twins
account for two-thirds of all twin pregnancies and have the lowest
risk of all twin pregnancies, as each baby has its own placenta and
sac. Because the babies are fertilised by different sperm, they may
be different sexes, have different blood types, and look entirely
different - on the other hand, they may look extremely similar,
just like any siblings.
One-egg twins are traditionally referred
to as "identical", a rather imprecise term which annoys some twins.
The technical term is "monozygotic", which can be preferable as it
focuses on the twins' biology rather than their looks. After all,
"identical" twins are not usually identical! While they do have
identical DNA, their genes may express in different ways. These
epigenetic differences tend to increase with age due to lifestyle
differences and environmental triggers; as a result, twins who
looked identical at age six may look markedly dissimilar at
Monozygotic twins come in four categories,
depending on how soon the egg splits after fertilisation. Between
one and three days after conception, the split is so early that
each twin develops its own amniotic sac and placenta. Called
dichorionic/diamniotic twins (DCDA), these babies tend to have a
low risk profile similar to that of dizygotic twins.
When the egg splits between four and eight days
after conception, the result is monochorionic/diamniotic (or MCDA)
twins. These babies share a placenta, but have separate sacs. MCDA
twins are the most common type of monozygotic twins, occurring in
60-70% of instances.
A late split (between eight and 13 days
after conception) results in a high-risk form of twin pregnancy
known as monochorionic/monoamniotic (MCMA) twins. Sharing both a
sac and a placenta, these twins are usually monitored closely in
late pregnancy to ensure that their movements do not tangle the
umbilical cords around each others' necks.
Both MCDA and MCMA twins are also at risk
for twin-to-twin transfusion syndrome (TTTS), a disease of the
placenta in which blood is shunted unevenly between the babies via
the connecting blood vessels in their shared placenta.
Finally, a very late split (more than 13
days post-conception) can be partial and result in conjoined twins.
The term "Siamese twins" refers to a famous pair of Thai conjoined
twins, Chang and Eng Bunker, and is not considered appropriate
today. Such twins are extremely rare - estimates range from one in
30,000 to one in 200,000. Conjoined twin pregnancies are the
riskiest of all, as many conjoined twins have severe disabilities.
There are examples of twins being joined in many different ways;
the most common include thoracopagus (stomach-to-stomach),
pygopagus (back-to-back), craniopagus (head-to-head) and
thoraco-omphalopagus (chest-to-chest) twins. Conjoined twins always
share at least one organ, and the possibility of separation after
birth depends on the degree of interdependency of the babies' organ
Telling the difference
In the case of boy/girl twins, it is generally safe to assume they
are not monozygotic! With same-sex twin pairs, it can be harder to
tell. Having a single placenta is not a good indication of
monozygocity, as the separate placentas of dizygotic twins can fuse
during pregnancy. Nor are looks any indication! Case in point:
Mary-Kate and Ashley Olsen, so identical as toddlers that they
played the same character on TV, are actually fraternal
If twins look alike and have the same
blood type, it may not be possible to determine zygosity without a
DNA test. Many twins are quite happy not knowing what "kind" they
are. However, knowing zygosity can be useful in situations
requiring organ or blood donation.
If by this point you're thoroughly
confused, don't worry. I asked a friend of mine the other day
whether her twins were MCMA, MCDA, or DCDA, and her eyes stared
into middle distance for some time before she replied, "They shared
a... Hey kids, we have to go. It was a... Don't hit your brother.
They both had a... Hang on. Was it a placen... no, that's not your
jacket. What were you asking?"
"Never mind," I said and wandered off with my
singleton baby, feeling the familiar sense of awe and fearful
respect that comes from being around mothers of twins. But I
couldn't help glancing over my shoulder... Awwww. Twins.
Sarah Tennant lives in Hamilton with her husband
Dominic and her daughter Rowan (one).
References and further
* Bigalk, Kris. "rare forms of twinning."
* Chitale, radha. "Woman with two wombs gives birth to twins". ABC
news Medical Unit (3 March 2009).
* Edmonds LD, Layde pM. "Conjoined twins in the United States,
1970-1977." Teratology 25 (1982): 301-08.
* Schmidt, r, et al. "Monozygotic twins discordant for sex."
Journal of Medical
Genetics 13.1 (February 1976): 64-68.
* Zach, Terence. "Multiple births."
* If you're still confused about chorionicity and zygosity, or
want to know more about TTTS, visit the pregnancy section of this website. There's
even a week-by-week pregnancy guide for
mums-to-be expecting multiples.
As seen in OHbaby!
magazine Issue 6: 2009
Subscribe to OHbaby! magazine