The rate of multiple pregnancies is showing a significant increase in Western countries. Twin gestations should be considered a high-risk condition because they are responsible for a disproportionate amount of overall perinatal morbidity and mortality due to fetal and maternal complications often related to impaired utero-placental function.
This is an observational study of uterine artery (UtA) Doppler velocimetry in women with dichorionic (DC) and monochorionic (MC) twin pregnancies.
We aim to discover if transabdominal UtA pulsatility index (PI) differs between MC diamniotic and DC twins and if it is useful to predict pregnancy complications compared to singletons .
Our secondary outcome is to create a reference chart of normal range for UtA Doppler parameters (pulsatility index and resistence index) in twin pregnancies for clinical use. We assume to include uncomplicated twin pregnancies, available for full pregnancy follow-up. Uterine Doppler velocimetry will be performed at first, second and third trimester and compared for demographic characteristics, mean UtA PI, presence of notch, development of preeclampsia, fetal growth restriction (FGR), placental abruption, intrauterine fetal death and preterm delivery.
Sensitivity, specificity, positive predictive value, negative predictive value, and likelihood ratio will be calculated for mean PI >95th percentile, presence of notch, and presence of either elevated PI or notch.
UtA measurements will be performed by color and pulsed Doppler imaging. The uterine artery PI and RI will be calculated as a mean of both sides. Reference ranges will be constructed related on gestational age.
Our purpose is to give to the sonographers updated information and a high sensitive tool for identifying properly low- and high-risk groups, and to better manage twin pregnancies.
Our study is going to present new reference charts of uterine artery PI and RI in MC twin pregnancies. The data that will be used to construct the reference ranges represent our measurements in clinical practice.
To date, scientific literature has not reference tables with maternal Doppler velocimetry values specific for MC pregnancies. In 2011 by Geipel et al. drawn up chart for DC twin pregnancies. They are the only available tool to sonographers.
As multiple pregnancies constitute a high-risk group for antenatal care, evaluation of uterine artery Doppler indices could be useful in the prediction of utero-placental dysfunction. In recent studies the performance of uterine Doppler investigation in the prediction of pre-eclampsia and IUGR was slightly poorer for twins than for singletons (Yu 2002, Geipel2002). However, it could be speculated that this method also identifies pregnancies that are at the highest risk for adverse outcome and that might benefit from increased antenatal surveillance.
It is not sure if normal UtA Doppler values for any gestational age are identical for MC and DC pregnancies. Several studies in literature showed no differences between twin pregnancies in terms of PI and RI indices, but recently in 2019 Masini et al. conducted a retrospective observational trial showing that there are differences in uterine arteries ranges in MC and DC twin pregnancy. Pulsatility and resistance index seem to be lower in BC than in MC, unlike previously thought.
It seems also shared by all authors that the sensitivity and reliability of these Doppler measurements in case of twin pregnancies is lower than in singelton pregnancies predicting the risk of unfavorable obstetric outcomes, but is unknown the real predictive power.
Given the increased number of twin pregnancies that obstetricians have to face, due to older maternal age at the moment of conception and the higher use of medically assisted fertilization techniques, it is necessary to promote studies that increase knowledge and find new screening tools to prevent complications.
Twin pregnancies represent one of the greatest risk conditions, simultaneously for the mother and the baby. Among the possible maternal complications, we list: anemia, gestational hypertension / preeclampsia, preterm birth / PPROM, placenta previa, placental abruptio, postpartum hemorrhage. Fetal complications include: Congenital anomalies, IUGR, Prematurity. There are typical complications of MC pregnancy (Twin-Twin Transfusion syndrome or TTTS and IUGR [Intra Uterine Growth Restriction] selective) which must be treated and managed in Fetal Medicine Centers with specific experience. (SIEOG2011)
The incidence of fetal loss is significantly higher in MC pregnancies than in BC, mainly for fetal losses in the second trimester. The incidence of neurological morbidity is also higher in MC pregnancies than in DC. This justifies different clinical and ultrasound monitoring protocols between MC and DC pregnancies, and the usefulness to follow monochorionic pregnancies in specialized centers.
We will collect data from twin pregnancy admitted to our high risk pregnancy unit as referring third degree center, to obtain information and to create reference charts with normal ranges of uterine artery Doppler parameters in particular in MC twin pregnancies.
The use of UtA reference ranges adapted to MC twin gestations will be more appropriate for identifying low- and high-risk groups.