This volume collates case material from over 500 autopsies (including embryos) and 1200 placentas from twins, triplets, and higher multiples with selected literature on human multiple reproduction. It emphasizes new concepts in pathophysiology and interprets less-than-ideal outcomes of multiple conception based on over 30 years experience in the field. Aspects of perinatal pathology unique to twins and higher multiples are emphasized, with practical suggestions for the service pathologist, including items of potential medicolegal significance. Controversial topics, such as the value of studies of concordance for diseases or malformations, and the significance of inter-twin anastomoses, are addressed and suggestions made for further investigation. A close working relationship between pathologists and their clinical colleagues is encouraged by discussion of clinico-pathological correlations, and the most recent discussions on mechanisms of twinning and genetic versus environmental influences on the development of twins are presented.

Pathology of Multiple Pregnancy

Objective: To report our experience with intentional puncture of the intervening membrane (‘septostomy’) for the treatment of the twin oligohydramnios-polyhydramnios sequence (TOPS)

Amniotic Septostomy for the Treatment of Twin Oligohydramnios-Polyhydramnios Sequence

Twin-to-twin transfusion syndrome presents in the mid-trimester with gross discordance in amniotic fluid volume, and complicates 10-15 per cent of monochorionic twins. Recent studies suggest a primary vascular basis in which a paucity of the bidirectional superficial anastomoses normally found in monochorionic twins is unable to compensate for haemodynamic imbalance resulting from unidirectional transfusion along deeper arterio-venous anastomoses. It is associated with high rates of perinatal mortality from ruptured membranes, hydrops and growth restriction, and a significant morbidity from cardiac and neurological sequelae in particular. Serial aggressive amnioreduction is the current treatment of choice, with survival in around two thirds of cases. In the remaining third, with features suggesting a poor outcome, selective fetocide may have a role. Current attempts at vascular ablative therapies have been associated with inferior survival rates, but the long term therapeutic goal remains the identification and ablation of the shared chorionic vasculature.

The aetiology and management of twin-twin transfusion syndrome

Twin-twin transfusion syndrome (TTTS) complicates one in five monochorionic pregnancies and is generally associated with high mortality and morbidity. One twin (the recipient) grows appropriately and has polyhydramnios while the other (the donor) may have a reduced growth velocity and severe oligohydramnios. The disparities in amniotic fluid volumes represent differences in fetal urine output. These differences occur secondary to hemodynamic changes, in which the vascular arrangement of placental anastomoses in TTTS leads to unidirectional flow from the donor to the recipient twin. A better understanding of the pathophysiology may contribute to improved management of this morbid condition. We studied three consecutive prospectively diagnosed stillborn twin pairs affected by early-onset TTTS. Renin gene expression was studied in sections of fetal kidneys with immunocytochemistry using a renin antiserum and with in situ hybridization using riboprobes complementary to renin mRNA, and renin-secreting cells (RCC) were counted. The overall maturation of the renal cortex was assessed by the percentage of immature glomeruli. The donor twin kidneys were smaller than those of the recipients, but the maturation of the renal cortex was not significantly different (28.2% immature glomeruli in the donor and 24.4% in the recipient kidney). The donor kidney showed increased renin gene expression with hyperplastic juxtaglomerular apparatuses (JGAs) that contained excess RCCs (median 20.02 [25th-75th centiles, 5.4, 25.1 RCCs per 100 glomeruli]). In contrast, the recipient kidney was virtually devoid of these cells (0.04 [0, 0.36] RCCs per 100 glomeruli; P < 0.05). In the donor kidney, increased renin release may, by a local action, contribute to renal vasoconstriction and oliguria. Increased renin and/or angiotensin II in the blood passing through the placental anastomoses may, by an endocrine action, suppress renin synthesis in the recipient kidney, thereby increasing renal blood flow and causing polyuria and polyhydramnios. These changes in the renal RAS could thus contribute to the pathogenesis of TTTS. The renal renin changes noted here may represent a contributory or compensating mechanism, the success of which may dictate the overall survival of the twin pregnancy and allow better understanding of the pathophysiology and perhaps therapy that may be employed in this condition.

Renin gene expression in fetal kidneys of pregnancies complicated by twin-twin transfusion syndrome.

Twin-twin transfusion syndrome (TTTS) is attributed to trans-anastomotic transfusion between twins. Anastomoses are ubiquitous in monochorionic (MC) placentae, yet TTTS develops in only 15%. Although ex vivo and in vivo studies fail to identify a unique anastomotic signature, TTTS placentae are typically associated with an imbalance in unidirectional arteriovenous anastomoses with absent bidirectional anastomoses. Doppler detection of an artery-artery anastomosis reduces the chance of TTTS, whereas, in those that develop the disease, it improves stage-independent survival. Selective laser is often curative, but an increasingly recognized risk of persistent or reverse TTTS may be attributable to atypical arteriovenous anastomoses not identifiable from the chorionic plate. Simple dysvolaemia fails to explain several phenotypic features, including haematological concordancy, recipient hypertension, and reversibly absent end diastolic flow in the donor. The renin-angiotensin system is upregulated in the donor and downregulated in the recipient's kidneys, while paradoxically raised renin levels in the recipient may contribute to raised afterload along with endothelin. Although research is limited in humans by therapy and the lack of a suitable experimental model, further studies of placental and vascular pathophysiology may not only refine current treatment modalities but may also, in addition, suggest further avenues for downstream management such as genetic predisposition testing or pharmacological intervention.

Insights into the pathophysiology of twin-twin transfusion syndrome

In twelve twin pregnancies with normal amniotic fluid volume, the urine output of each twin was lower than in fetuses from singleton pregnancies, and the combined urine output of both twins was between the 50th and 95th centile for singletons. In three twin pregnancies at 21-24 weeks of gestation with acute polyhydramnios, presumed to be due to the twin-twin transfusion syndrome, the urine output of the smaller fetus was zero and that of the larger was above the 95th centile for normal singleton pregnancies. These three pregnancies were managed by repeated amniocenteses and rapid drainage of large volumes of amniotic fluid. With advancing gestation, there was a tendency for normalization of urine output in the twins.

Fetal urine production in normal twins and in twins with acute polyhydramnios

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Fetal urine production in normal twins and in twins with acute polyhydramnios