We prospectively evaluated for the first time the concordance of the risk of NRM between twins with non-invasive quantitative ultrasound lung texture analysis, obtaining 97.4% (Group 1), 73.5% (Group 2) and 88.4% (Group 3) concordance in the risk of NRM. Indeed, the risk of NRM was discordant between twin pairs in 2.5% (1/39), 26.5% (13/49) and 11.6% (5/43) in Groups 1, 2 and 3, respectively. We found 97.4% (77/78) of fetuses with high-risk results in Group 1, 27.6% (27/98), Group 2 and 5.8% (5/86) Group 3. These results achieved good concordance in Groups 1 and 3, being moderate in Group 2. This finding may be helpful in clinical practice since frequent obstetric diagnoses impel delivery planning in twin pregnancies in the late preterm and early weeks of gestation. In such circumstances delaying delivery could be ambiguous, and information of lung maturity may help to weigh maternal and fetal risks. Our univariate logistic regression analysis showed that spontaneous conception, gestational diabetes mellitus and gestational age <32 weeks were associated with a high-risk of NRM in Group 2. Variables associated with high-risk results were not found in Groups 1 or 3.
A decade ago, the prediction of fetal lung maturity relied almost solely on laboratory tests of amniotic fluid obtained invasively (amniocentesis). Previous studies using these tests have described similar differences in FLM to those of our study at a given gestational age and many gestational factors had been proposed as predictors of FLM concordance.
Regarding gestational age and chorionicity, Mackenzie performed amniocentesis in 92 pairs of diamniotic twins and found that at extremes of gestational age (28, 29, 36 and 37 weeks), twins were 100% concordant in lung status, while lung concordance was significantly lower in twins of 33-35.9 weeks, and even lower at 30-32.9 weeks. The influence of chorionicity, estimated fetal weight discordance and fetal gender concordance were not predictive of lung maturity22. In a study including 42 pairs of twins, Leveno et al. reported a high correlation between lecithin/sphingomyelin (L/S) ratios between twins A and B which was not related to birth order, infant sex, birthweight discordance, or zygosity23. Following amniocentesis in 47 twin pregnancies (26 to 36 weeks), Winn et.al, reported that FLM was biochemically comparable between twins A and B24, being in line with our results showing concordant results in 97.4%, 73.5% and 88.4% of twins in Groups 1, 2 and 3, respectively. Similar to our results, McElrath, et al. found that after 31.0 weeks, twin gestations had significantly higher levels of TDx (surfactant/albumin ratio assay) of FLM values25. Another study by Whitworth et al. showed that when L/S ratios were stratified by gestational age at amniocentesis, the mean percentage difference of the ratios was 25% at ≤32 weeks and 16% at >32 weeks. In all the sets of twins, in which the ratios were discordant, the gestational age at amniocentesis was ≤32 weeks26. Dobbie et al. explored the correlation of biochemical ratios [L/S, phosphatidyl glycerol/sphingomyelin (PG/S) and phosphatidyl inositol/sphingomyelin (PI/S)] of FLM tests in 32 pairs of twins. They found a weak correlation for L/S ratio but a much-improved correlation for PG/S and PI/S. Concordance was greater in monochorionic pregnancies 1. In a large study of 132 dichorionic and 125 monochorionic twins, Tsuda et al. examined the amniotic lamellar body count (LBC) and found that the concordance of the LBC results for the cut- off value was also significantly lower in dichorionic than in monochorionic twins27 .
We found no association of fetal gender with the QuantusFLMÒ results, but we did not test the results with the outcome at birth. However, the presence of RDS at birth was more prevalent in male newborns, similar to the studies by Mulla et al.28 and Steen et al.29.
Maternal diabetes has commonly been reported as a factor related to FLM30,31 and FLM tests32,33. It has also been associated with mixed results such as more frequent admission to neonatal intensive care units and greater birthweight but not with greater respiratory morbidity in twin pregnancies34,35. In our study gestational diabetes was a statistically significant predictor of high-risk results in Group 2; however, the number of cases was limited. This can be expected in these weeks of transition from pulmonary immaturity to maturity, since this association was not found in Group 3.
Assisted reproductive techniques have been associated with a negative effect on twin pregnancy outcomes36-40. We found no relationship between these techniques and FLM to explain why IVF seemed to be a protective factor in this group. Tsuda et al. evaluated the impact of fertility treatment on NRM in twin pregnancies using amniotic LBC and found no statistically significant associations between fertility treatments and the rates of RDS/TTN4142.
The aim of the present study was not to test performance but rather to explore the concordance of the high- or low-risk of NRM in prospectively followed twin pairs. This study showed a relevant number of cases of fetal lung immaturity, at least in one of the fetuses, in the gestational age ranges of Groups 2 and 3, which should be considered if delivery is expected. While in Group 2, 59.2% of pairs were already at low-risk, 11.6% in Group 3 remained at high-risk, in line with studies in which FLM was evaluated by invasive techniques.
The main strength of this study is first, the fact that a non-invasive technique was used to evaluate the individualized risk for NRM for each twin; and secondly, our study included a larger number of pairs than the studies by Leveno et al.23, Mackenzie22 and Winn et al.24, who included no more than 92 pairs of twins. However, some limitations are of note: respiratory morbidity was not clinically addressed immediately after the test. However, the study was designed to evaluate the concordance of the test in utero between twins to help clinical decision-making. Indeed, a multicenter study demonstrated that QuantusFLMÒ has an accuracy of 86.5%, like that of invasive techniques although a further study to evaluate this technique in twins is forthcoming.
In conclusion, there was good concordance of the risk of NRM between twins <30.0 weeks and >34.0 weeks. From 30.0 to 33.6 weeks 26.5% of the twin pairs had discordant results. Gestational age ≥32 weeks and IVF status were found to have a protective effect on the risk of NRM in this group. In 11.6% of pregnancies in the group above 34 weeks, at least one fetus of these pairs, showed high-risk of NRM. This information may be helpful to plan and guide obstetricians in the clinical decision-making process.