WOMENS-HEALTH Messages for September, 1998: Re: monochorionic twins

Re: monochorionic twins - does anyone know anything?

From: Thomas (anonymous@obgyn.net)
Tue, 29 Sep 1998 04:43:09 -0500 (CDT)

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In reply to: Sandra: "monochorionic twins - does anyone know anything?"

This is an interest of mine.

2/3rd of twins are dizygotic (non-identical, fraternal)and result from two ovae (eggs) being fertilised by 2 sperm. They form two fetuses, 2 placentae (which may fuse), and 2 sets of sacs - Dichorionic, Diamniotic twins (DCDA). The inner sac is called the amnion, the outer sac is called the chorion.

1 /3rd of twins are monozygotic (identical)and result from one ovum being fertilised by one sperm. The resulting embryo splits at a very early stage to cause identical twins. 1/3 of these monozygotic twins form so early that they form DCDA twins similar to dizygotic pairs. 2/3rd share the outer membrane called the chorion and the placenta but have seperate sacs formed by the amnions. Very rarely, they can split much later and cause monoamniotic twins or conjoint twins (Siamese) if this cleavage occurs even later.

Monoamniotic & Siamese twins are very rare. Monoamniotic twins share a sac. Being a typical obstetrician one of my best friends had some. The risk is that the babies cords will tangle in the womb causing them to die. The prognosis is poor but in specialist centres as high as 50%. Usually the women is given a drug called Sulindac to decrease the fetuses movements and the amount of amniotic fluid. Delivery normally occurs by C/S at 32 weeks. It is highly unlikely you have monoamniotic twins as they are rare and you would know about this by now. If you do, you should be managed is a subspecialist fetal medicine department.

Monochorionic diamniotic (MCDA) twins are not uncommon as they occur in 2/3rd of monozygotic twins (1/3rd of all twins). If you have had IVF you are still at risk of having MCDA twins even if 2 ovae were implanted as cleavage is more common after IVF. MCDA twins are diagnosed by scan by a thin septum between the 2 twins and the abscence of something called the 'twin peak' or 'lamda' sign which is a tongue of placental tissue sticking in between the 2 sets of membranes in DCDA twins. Diagnosis is only about 90% even if made at an early scan when it is more accurate. Therefore it is essential that the twins are sexed at 18 weeks as if they are separate sex they are definitely dizygotic (non-identical) and therefore DCDA.

MCDA are at greater risk of something called the twin twin transfusion syndrome (TTTS)(or feto feto transfusion syndrome (FFTS)). This is where one baby receives more blood resulting in one plethoric, large twin with xs fluid and one small anaemic twin with less fluid. This can result in a need for something called amnio-reduction where fluid is taken off or early delivery is required. It is therefore essential that someone with MCDA twins receives regular scans from about 24 weeks. I perform them every 2 weeks but some doctors do them every 3 or 4 weeks. They are also at risk of sudden TTTS in labour. As a result many obstetricians recomend elective C/S. Others, such as myself, recomend continuous monitoring with a low threshold to C/S. In the past obstetricians offered laser treatment to communicating vessels in TTTS. However, we now know that a woman with MCDA is at more risk of TTTS if she has only a few vessels than if she has a large number.

There are other implications of having MCDA twins. If one twin dies then there is a 50% risk of the otherone dying which implies that delivery needs to occur soon. Furthermore, if left, there is risk of brain and kidney damage.

Finally, if a woman has DCDA twins and needs an amniocentesis it is necessary to take a sample from both sacs. If they are MCDA twins they are identical and therefore only need one needle puncture thus reducing the risk of miscarriage.

Hopw this helps

--
Dr Thomas Ind MB BS MD MRCOG
St George Hospital
Kogarah
Sydney
NSW 2217
AUSTRALIA

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