Longitudinal blood flow in shared (arteriovenous anastomoses) and non-shared cotyledons in monochorionic placentae.

OBJECTIVES: In this study we aimed to quantitate monochorionic twin placental blood flow in vivo through arterio-venous anastomoses (AVA) and corresponding vessels within normal cotyledons. METHODS: The topography of chorionic plate vasculature was mapped using colour Doppler in ten monochorionic diamniotic twin (MCDA) pregnancies. Cotyledonary flow was derived by insonation of chorionic veins draining normal (n=10) and paired control shared cotyledons (n=10). Venous volume flow was calculated from five determinations of vessel diameter and three of time average mean velocity (TAMV). Measurements were repeated every 2-4 weeks from 18 until 32 weeks' gestation. RESULTS: Blood flow through non-shared and shared cotyledons increased with gestation (p<0.0001). Median flow at 28 weeks through shared cotyledons was 16 ml/min (15-21) (median, interquartile range), lower than in shared cotyledons (31, 25-35) (p<0.001), as was median volume flow across gestation calculated as area under the curve (shared cotyledons 126 (122-167), control cotyledons 269 (214-274), p=0.01). However, velocity was similar, with the difference due to smaller vein diameters draining shared compared to normal cotyledons (mean 3.6mm (SD 0.8) vs. 4.5mm (0.8), p=0.004). Ex vivo quantitation of insonated cotyledons and of all cotyledons confirmed the difference in vein diameter in the placentae studied. CONCLUSIONS: Blood flow through shared cotyledons was lower across gestation than through paired normal cotyledons in the placenta studied due to the smaller diameter of the AVA vessels. The size of AVAs rather than simply their presence and direction may contribute to determining transfusional imbalance in monochorionic twins.

Histomorphometric characterisation of shared and non-shared cotyledonary villus territories of monochorionic placentae in relation to pregnancy complications.

OBJECTIVE: Theoretical estimates and physiological inferences suggest that the structure of a shared cotyledon differs from a non-shared cotyledon. The aim of this study was to characterise the histomorphometry of terminal villi in shared and non-shared cotyledons in monochorionic placentae, both from uncomplicated twins and from those with twin-twin transfusion syndrome (TTTS) or discordant growth restriction (DeltaIUGR). METHODS: Forty-one monochorionic placentae from Caucasian non-smokers were obtained at caesarean section. Their vascular anatomy and placental territories were ascertained by dye injection. After fixation, full thickness histological blocks were obtained by systematic random sampling from each twin's territory and the shared cotyledons. Fifty randomly selected terminal villi were assessed for: (i) median villus diameter (ii) median villus capillary diameter (iii) median fetomaternal diffusion distance (iv) median no. of capillaries/villus (v) degree of vascularization (median percentage cross-sectional area of terminal villi occupied by capillaries) using a stage micrometer and image analysis programme. The histomorphometric findings were then correlated with birthweight discordance, placental territory discordance and DeltaAVAs (no. of AVAs from smaller twin (donor) to larger twin (recipient) minus no. of AVAs from larger to smaller twin). RESULTS: Histomorphometric variables were similar in shared and non-shared cotyledons of uncomplicated MCDA twins. However, the median diameter of terminal villi in shared cotyledons in DeltaIUGR and TTTS placentae was significantly smaller [51.2 microm (48.2-58.3), p<0.001 and 52.6 microm (53.1-50.4), p<0.001], and had a similar number of smaller capillaries, larger fetomaternal diffusion distance and reduced vascularization compared to non-shared IUGR and TTTS placentae. However, Deltadiameter (defined as the difference between median diameters of terminal villi in large minus small twins' territories) rose with increasing birthweight discordance (Pearson correlation coefficient=0.82, p<0.001). Multiple linear regression analysis revealed that Deltadiameter was influenced by placental territory discordance (p<0.001) and birthweight discordance (p<0.01): log10 Deltadiameter=1.38+(0.01 x birthweight discordance)+(0.56 x log10 placental territory discordance) (R2=0.82, p<0.001), but there was no significant relationship with DeltaAVA and AAA. In the TTTS group, Deltadiameter correlated significantly with DeltaAVA only: log10Deltadiameter=1.44+(0.02 x DeltaAVA) (R2=0.3, p<0.001). CONCLUSIONS: This is the first study to characterise the histomorphometry of shared and non-shared cotyledons in MC twins. The findings suggest that abnormal placentation, rather than placental vascular anatomy may be responsible for DeltaIUGR in MC twins, whereas TTTS arises from imbalance in interfetal transfusion with resultant differing terminal villus histomorphometric features in donor, recipient and shared cotyledons.

Increased latency of absent end-diastolic flow in the umbilical artery of monochorionic twin fetuses.

OBJECTIVE: To determine if absent end-diastolic flow (AEDF) in the umbilical artery (UA) has a longer latency in monochorionic (MC) twin fetuses compared to singleton or dichorionic twin (DC) fetuses. METHODS: One hundred and eight pregnancies with a fetus with AEDF were reviewed: 47 MC and 17 DC twin pregnancies and 44 singletons. Because twin-twin transfusion syndrome (TTTS) is a potential confounder when determining latency, subgroup analysis was also performed on the 21 MC affected pregnancies without TTTS. Latency of AEDF (in days) was defined as the difference between the gestational age at diagnosis of AEDF and gestational age at delivery or intrauterine death. RESULTS: Latency was similar in MC twins (median, 39 days) and DC twins (30 days) but longer compared to singletons (11 days; P = 0.0001). After excluding pregnancies with TTTS, latency in non-TTTS MC twins (54 days) was longer than in both singletons and DC twins. This was due to an earlier gestational age at AEDF in non-TTTS MC twins of 20 weeks compared to 27 weeks in both singleton and DC twins because median gestational age at delivery was similar in MC twins, DC twins and singletons. CONCLUSIONS: The latency period of UA AEDF is longer in MC twins than in singletons. Our data suggest that in MC twin fetuses without TTTS, AEDF begins earlier and lasts about twice as long as in DC twin fetuses, which is consistent with placental insufficiency not being the sole factor mediating abnormal UA waveforms in MC placentation. This observation is important in counseling and managing twin pregnancies discordant for AEDF.

Characterisation of deep arterio-venous anastomoses within monochorionic placentae by vascular casting.

OBJECTIVE: To characterise arterio-venous anastomoses (AVA) in monochorionic (MC) placentae and determine (i) whether shared cotyledons lie beneath the co-termination of an artery from one twin and a vein to the contralateral twin and (ii) whether all AVA can be detected by visual inspection of the chorionic plate. METHODS: Vascular casts were made of 15 MC placentae. The number of typical AVAs suspected visually before digestion was compared with the number of AVAs identified after acid digestion. RESULTS: Thirty-three of 67 (49%) suspected typical AVAs were confirmed as typical after casting. There were five false positives and no false negatives. The remainder were classified as atypical AVAs, found in > or =90% of MC placentae. Type I (small vascular connections between two apparently normal cotyledons not seen before casting) and Type II (shared cotyledons arising within larger apparently normal cotyledons) atypical AVAs were found in 53% and 73% of placentae, respectively. CONCLUSIONS: Only half the shared cotyledons in MC placentae are characterised by co-termination of an artery and vein on the chorionic plate. We report the existence of deep anastomoses beneath the chorionic plate that cannot be visualised by chorionic plate inspection. These findings have implications for laser treatment of twin-twin transfusion syndrome.

Transmitted arterio-arterial anastomosis waveforms causing cyclically intermittent absent/reversed end-diastolic umbilical artery flow in monochorionic twins.

OBJECTIVES: To characterize the phenomenon of retrograde transmission of arterio-arterial anastomosis (AAA) interference patterns on umbilical artery (UA) waveform by (a) documenting the periodicity, (b) correlation with in vivo and in vitro demonstration of AAAs and (c) reproducing these patterns by computer modelling. METHODS: Monochorionic twins (MC) twins underwent placental and umbilical Doppler studies. AAAs were sought by pulse wave Doppler of their bi-directional interference pattern and confirmed by postnatal injection studies. The periodicity of transmitted patterns in the UA was determined. Determinants of the transmitted patterns were ascertained by computer modelling of physiological and fetal variables. RESULTS: Among 83 prospectively studied MC twin pregnancies; a transmitted pattern was observed in 6 (7 per cent) patients for 15-114 days. This was found in 20 per cent (6/30) of smaller MC twins discordant for growth restriction but in no appropriately grown twins. It was only observed in association with AAAs validated both in vivo and in ex vivo. Computer modelling demonstrated that this pattern could be reproduced by summating end diastolic flow with a high pulsatility index in the UA in the presence of a large AAA. Consistent with this, MC twins with a transmitted pattern had larger AAAs (median diameter 4.3 mm interquartile range 4.1-5.2) compared to MC twins discordant for intrauterine growth restriction (2.1 mm interquartile range 1.5 to 2.8) (P<0.05) without a transmitted pattern. Perinatal mortality was similar in the fetuses with and without transmitted patterns (0/12 vs. 2/48 P=0.7).

Outcome of twin pregnancies complicated by single intrauterine death in relation to vascular anatomy of the monochorionic placenta.

The objective of this study was to determine the relationship between the type of placentation, vascular anatomy of the monochorionic (MC) placenta and the perinatal outcome of the surviving twin following a single intrauterine fetal death (IUFD). In this retrospective study, 92 twin pregnancies complicated by a single intrauterine death were identified from three tertiary referral centres [50 MC and 42 dichorionic (DC)]. Antenatal and neonatal data as well as information on the chorionicity, vascular anastomoses, and autopsy findings were also obtained. The percentage risk of IUFD (26 versus 2.4; P < 0.001), anaemia (51.4 versus 0; P < 0.001) and intracranial lesions at birth (46 versus 0; P < 0.001) was greater in MC than in DC twins. In MC twins without twin-twin transfusion syndrome (TTTS), perinatal mortality was higher in the group with superficial arterioarterial (AA)/venovenous (VV) channels than those with only multiple bidirectional arteriovenous (AV) anastomoses (12/15 versus 0/8; P < 0.001). However, in the TTTS pregnancies (n = 26), perinatal outcome of the surviving twin was dependent on whether the recipient (n = 16) or the donor twin (n = 10) died first. Incidence of IUFD (9/16 versus 0/10; P < 0.001), severe anaemia (7/7 versus 1/10; P < 0.001) and intracranial lesions at birth (6/7 versus 2/10; P < 0.001) was common in pregnancies where the recipient twin died first. In the TTTS group, unidirectional AV anastomotic channels were found in all but two placentae. In conclusion, this study suggests that the outcome of twin pregnancies complicated by IUFD is dependent on the type of vascular anastomoses and TTTS.