Determination of amnionicity and chorionicity is critical in the antenatal management of twins and should be done in the first trimester.
Chorionicity is important for managing structural anomalies, aneuploidy, the aetiology of fetal growth and/or fluid discordance, early diagnosis of twin-to-twin transfusion syndrome, and the management of a surviving twin following intrauterine demise.
There is a high mortality and morbidity of monoamniotic twins.
Monochorionic means 1 shared placenta.
Dichorionic means 2 placentas (twins/triplets).
Trichorionic means 3 placentas (triplets) and so on.
The amnion may be around 1 of the fetuses or there may be a second or third foetus within the same amniotic sac. It is important to turn the gain up to see if there is a thin membrane between the fetuses.
Labelling Foetal number
When labelling the fetuses it is important to be consistent so that follow up scanning does not mistakenly change the order.
The first twin, labelled Twin A or Twin 1, is closest to the cervix. The second fetus is the fetus adjacent to fetus 1 and next closest to the cervix, etc.
The number of yolk sacs may help diagnose the amnionicity of the twins.
When 2 yolk sacs are seen in the extra-embryonal coelom, the pregnancy will be diamniotic, while a single yolk sac in most cases indicates mono-amniotic twins.
A single yolk sac seen when there are two embryos should prompt a follow-up first trimester scan to definitively assign amnionicity.
Twin Peak/Lambda Sign/Chorionic Peak
The lambda sign λ is the symbol for the eleventh lower-case letter in the Greek alphabet. It refers to the triangular projection of chorion between the layers of inter-twin membranes.
The triangular shape is shown upside down below.
It is best seen between 10- 14 weeks gestation. If seen it is highly suggestive of dichorionic diamniotic twins (DCDA). It is also known as the twin peak sign because the fusion of the placentas looks like the appearance of a mountain top between the two sacs.
Lambda/Twin Peak/Chorionic Peak sign
Lambda λ is the symbol for the eleventh lower-case letter in the Greek alphabet. The triangular shape is shown upside down here.
Absence of sign
These twins at 6 weeks have 2 separate gestational sacs (ie diamniotic, dichorionic).
Monochorionic Diamniotic twins (MCDA)
These twins share a placenta which is posteriorly positioned but have an amniotic membrane separating them.
At this early stage (6 weeks) it is difficult to ascertain the chorionicity and amnionicity. At 10-12 weeks gestation it is more accurate.
Uncertain triplet chorionicity at this early stage.
In this gestational sac there are 2 fetuses (A and B) within 1 amnion and and a third fetus within another amniotic sac. At this early stage it is very difficult to assess for chorionicity but it is potentially MCDA or DCDA or DCTA triplets.
Look for the number of yolk sacs
The number of yolk sacs may help diagnose the amnionicity.8 When 2 yolk sacs are seen in the extra-embryonal coeloma, the pregnancy will be diamniotic, while a single yolk sac will in most cases indicate monoamniotic twins. A single yolk sac seen when there are dual embryos should prompt a follow-up first trimester scan to definitively assign amnionicity.
Trichorionic Triamniotic Triplets (TCTA)
This sagittal view of the uterus show 3 individual gestational sacs.
These triplets will develop their own placenta. The thick dividing membranes show the “lambda sign”.
Each triplet is labelled A,B and C in order of the distance from the cervix.
Dichorionic Triamniotic Triplets (DCTA)
Two of the fetuses share a placenta with a thin amniotic membrane dividing them (DA) and the third triplet sits within its own gestational sac and therefore has its own placenta. Ths there are 2 placentas in total and 3 amnions – DCTA
Role of Ultrasound
To ascertain the gestational age, amnionicity and chorionicity of the fetuses as well as the viability.
If the scan is done before 10 weeks it can be difficult to determine both the amnionicity and chorionicity accurately.
A full bladder for an initial scan before proceeding to a transvaginal scan.
The highest frequency probe possible which allows penetration through to the posterior wall of the uterus at the lowest power level.