Placental Characteristics Within Monochorionic Twin Pregnancies

To determine the prevalence of intertwin vascular anastomoses (overall, artery-artery, artery-vein, vein-vein) on adverse perinatal outcome (e.g. stillbirth and neonatal death) and fetal complications (e.g. small for gestational age). Measures will be as overall percentages and also raw numbers which will be calculated in to odds ratio's when comparing anastomoses versus no anastomoses against the respective perinatal outcome and fetal complication

To determine the prevalence of abnormal cord insertion (marginal and velamentous) on adverse perinatal outcome (e.g. stillbirth and neonatal death) and fetal complications (e.g. small for gestational age, selective growth restriction). Measures will be as overall percentages and also raw numbers which will be calculated in to odds ratio's when comparing abnormal cord insertion versus normal cord insertion against the respective perinatal outcome and fetal complication

To determine the impact of placental sharing between each twin on adverse perinatal outcome (e.g. stillbirth and neonatal death) and fetal complications (e.g. birthweight discordance and selective growth restriction). Placental sharing will be defined as a percentage of overall share between each twin which sums up to 100%. The difference between each twin's share will be determine by minimising the larger percentage from the smaller percentage. The difference will then be compared to adverse perinatal outcomes and complications such as selective growth restriction using odds ratio's. Birthweight discordance will be defined as a percentage ( difference between the larger twin weight and smaller twin weight, divided by the larger twin weight). This will be compared to placental sharing using a graphical comparison of all placental sharing results, individually compared against each respective birthweight discordance.

To understand the frequency of abnormal cord insertion (marginal and velamentous) within monochorionic twins complicated by twin-twin transfusion syndrome following laser ablation and determine the impact on fetal outcomes such as stillbirth, neonatal death, recurrent twin-twin transfusion syndrome and twin anaemia-polycythaemia sequence. Results will be collected as overall number of placentas with abnormal cord (both marginal, velamentous and both) and those with normal cord insertion, and compared against the number of events in the respective fetal outcome. Results will be presented as odds ratio's.

To understand the frequency of residual vascular anastomoses within monochorionic twins complicated by twin-twin transfusion syndrome following laser ablation and determine their impact on fetal outcomes such as stillbirth, neonatal death, recurrent twin-twin transfusion syndrome and twin anaemia-polycythaemia sequence. Results will be collected as overall number of placentas with residual vascular anastomoses, alongside the individual types of anastomoses (artery-artery, artery-vein, vein-vein) and compared against the number of events in the respective fetal outcome. Results will be presented as odds ratio's.

To understand the impact of abnormal cord insertion (marginal and velamentous) on residual anastomoses development in monochorionic twins complicated by twin-twin transfusion syndrome undergoing laser ablation. Results will be collected as overall number of placentas with abnormal cord insertion and whether they have residual anastomoses versus those without abnormal cord insertion and if they have residual anastomoses seen. Results will be presented as odds ratio's.

To examine the impact of Quintero staging during laser ablation for twin-twin transfusion syndrome on the frequency of post-laser residual anastomoses. Quintero staging will be split in to the four stages and expressed as overall numbers versus the number of placentas with and without residual anastomoses. Results will be expressed as odds ratio's.

To examine the impact of placenta position during laser ablation for twin-twin transfusion syndrome on the frequency of post-laser residual anastomoses. Placenta position will be either anterior or posterior and expressed as overall numbers versus the number of placentas with and without residual anastomoses. Results will be expressed as odds ratio's.

The assess the intercord twin distance on fetal complications such birthweight discordance. The distance of the twin cords will be measured in centimetres. This distance will be divided by the distance of the overall placental length to form a number between 0-1. This will then be compared graphically against birthweight discordance, which will be presented as a percentage (difference between the larger twin weight and smaller twin weight, divided by the larger twin weight).

To determine the frequencies of abnormal cord insertion (velamentous and marginal) in monochorionic twin groups. This will be defined as percentages (number of cases with abnormal cord (velamentous, marginal and both) divided by the overall number) in each twin group (uncomplicated, twin-twin transfusion, selective growth restriction, twin anaemia-polycythaemia, TRAP, monoamniotic). Results will be compared to each group using P values.

To determine the frequencies of intertwin vascular anastomoses (overall, artery-artery, artery-vein, vein-vein) in monochorionic twin groups. This will be defined as percentages (number of cases with intertwin vascular anastomoses (overall, artery-artery, artery-vein, vein-vein) divided by the overall number in each twin group (uncomplicated, twin-twin transfusion, selective growth restriction, twin anaemia-polycythaemia, TRAP, monoamniotic). Results will be compared to each group using P values