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Objective To determine in vivo whether monochorionic pregnancies complicated by twin-to-twin transfusion syndrome are associated with absence of haemodynamically-compensatory arterioarterial anastomoses.Design Forty monochorionic pregnancies were prospectively recruited for an ultrasonographic survey of the chorionic plate using colour Doppler energy. Arterio-arterial anastomoses were identified by their characteristic bidirectional interference pattern on spectral Doppler. Angioarchitecture was confirmed by postnatal injection study.Setting Fetal medicine tertiary referral centre in London. Main Outcome MeasuresPresence of arterio-arterial anastomoses, development of twin-to-twin transfusion syndrome, survival.Results Arterio-arterial anastomoses were detected by colour Doppler energy in 2 1 pregnancies (53%), and there were no false positives. An arterio-arterial anastomosis was more commonly found in unaffected (n = 28) compared to pregnancies affected by twin-to-twin transfusion syndrome (n = 12), both by colour Doppler energy [20/28 (71%) vs 1/12 (8%); A = 63%, 95% CI 400/1A6%;,] and by postnatal injection study [25/28 (89?4) vs 3/12 (25%); A = 64%, 95% CI 37%-91%]. In pregnancies in which no arterio-arterial anastomoses were detected, a diagnosis of twin-to-twin transfusion syndrome was made in 58%, and the perinatal loss rate was 40%, compared with one case of twin-to-twin transfusion syndrome (5%) (P C 0.001) and a loss rate of 12% (P = 0.005) in pregnancies in which an arterio-arterial anastomosis was detected.Conclusion Twin-to-twin transfusion syndrome is associated with an absence of functional arterioarterial anastomoses in vivo in monochorionic twin pregnancies. This contributes to our understanding of the pathophysiology of twin-to-twin transfusion syndrome and confirms ex vivo studies demonstrating that twin-to-twin transfusion syndrome is associated with a paucity of superficial anastomoses. Prospective studies are indicated to determine the utility of colour Doppler energy for arterio-arterial anastomoses in predicting risk in monochorionic pregnancies.

Objective To characterize amniotic pressure (AP) in pregnancies with normal amniotic fluid volume. Design Observational study, mainly cross‐sectional. Setting Fetal medicine unit within a tertiary referral hospital. Subjects Patients undergoing transamniotic invasive procedures in whom amniotic fluid volume was subjectively assessed as normal on ultrasound. Those beyond 16 weeks with a deepest vertical pool on ultrasound <3.0 or >8.0 cm were excluded. Overall 194 pregnancies were studied on 232 occasions between 7 and 38 weeks gestation. Interventions Manometry readings referenced to the top of the maternal abdomen were obtained via a fluid‐filled line from the needle hub and either connected to a pressure transducer (n= 190) or held vertically against a ruler (n−42). Main outcome measures AP in mm Hg, AP corrected for gestational age (z scores), semi‐quantitative ultrasonic indices of amniotic fluid volume, clinical variables. Results AP in singleton pregnancies increased with advancing gestation (P<0.001), and the sigmoid‐shaped regression curve plateaued in the mid‐trimester. AP z scores were not influenced by volume‐related phenomena such as twin gestation, the deepest vertical pool, or amniotic fluid index, nor by maternal age, parity, gravidity, fetal sex, or subsequent spontaneous preterm delivery. Conclusions These findings suggest that AP is not principally determined by intrauterine volume. We speculate that AP, which reflects change in uterine tension as a function of radius, may instead be determined by gestation‐specific anatomical and hormonal influences on gravid uterine musculature. A reference range for AP has been constructed for use in amnioinfusion and amnioreduction procedures.

High pulsatility indices (PIs) and/or notches on the Doppler flow velocity waveforms of the uterine artery have been interpreted as indications of high placental flow impedance, and are known to be associated with poor fetal outcome. A software model of the uteroplacental blood path and its use to investigate possible interactions within the uteroplacental unit in more detail are described. Increasing transcotyledonary resistance to represent intervillous obstruction raised the cotyledonary core pressure and spiral artery PI. Increased spiral artery flow resistance, representing failed spiral artery invasion, reduced the cotyledonary core pressure and reduced the spiral artery PI. In vivo, such changes in cotyledonary core pressure would modify the transplacental water balance, promoting oligohydramnios for spiral artery invasion failure and polyhydramnios for villous obstruction. Both mechanisms increased the uterine and arcuate PI, but failed to produce a notch. It was found that notch formation depended on terms representing increased compliance (distensibility) of the uterine and/or arcuate artery walls, which have no direct effect on uteroplacental mean flow. The same mechanism steepened and increased uterine artery peak systolic flow, contributing to increased PI. The notch phenomenon seems to be an indicator of abnormal maternal artery wall status, independent of placental obstructive mechanisms, which can mask obstructive PI changes. Computer analysis of the frequency index profile should allow separation.