4.DISCUSSION
In this study, we mainly found that UA velocities were decreased with the severity of growth restriction and the UA-TAMXV was independent predicting factor of FGR. To our knowledge, this is the first study to assess associations between the UA velocities and growth disorders.
In this study, the absolute UA velocities is decreased with the severity of growth restriction, that’s to say, it is decreased in SGA group and further decreased in FGR group when compared with AGA group at 37 weeks, suggesting that the absolute UA velocities was associated with the feto-placental blood flow. As all of the fetuses enrolled in our study presented normal UA Doppler spectrums, our finding suggested that UA velocities, particularly UA-TAMXV and UA-PSV, might be more predictive of late-onset FGR and SGA than the conventional UA Doppler parameters, such as UA-PI and UA-S/D.
Previous studies have shown that abnormal fetal middle cerebral artery may occur in 20% of term SGA fetuses with normal UA Doppler results18 19. It has certain value for clinical management of FGR20, however, its role in predicting FGR with normal UA Doppler spectrums remains unclear. As described in previous studies, the placental volume blood flow could also be assessed by the umbilical venous velocities21 22. However, the umbilical venous velocities were prone to errors and still need to be standardized23. One longitudinal study demonstrated that the UA-TAMXV was best correlated with the umbilical vein volume blood flow14. Our study showed that UA-TAMXV was decreased in the FGR and was consistent with previous studies. In routine ultrasound examination, as the UA circulation Doppler has been generally used to assess the feto-placental blood circulation, the UA velocities can be easier and quicker to acquire than the umbilical vein velocities. As we know, when the placental blood flow resistance increases, the end diastolic blood flow decreases. Therefore, the UA-EDV might be absent and cannot be detected in severely growth-restricted fetuses, whereas the UA-TAMXV can still be readily detected in this condition. Our findings suggested that the UA-TAMXV might be of predictive value for the FGR. Therefore, the UA-TAMXV might be a more preferred parameter for evaluating the placenta volume blood flow as well as the degree of fetal ischemia.
Previously studies reported that fetal growth disorders are associated with an increased risk of APOs2 3. Unfortunately, no obvious APOs were found in our study. The possible might be as follows. Firstly, this negative finding may be related to the small sample size. Furthermore, all of the fetuses in our study had normal UA Doppler spectrums, which indicates that the fetus was not seriously compromised. Lastly, the timing for delivering in SGA fetus is at 37 weeks or so rather than terminating the pregnancy until the fetus is severely damaged. A large study reported a significantly increased risk of fetal death in SGA delivered >37 weeks compared to those delivered at 37 weeks24. Another study including 92,218 singletons found that the fetal death rates were lower in detected than those in undetected FGR2. It follows then that the key to preventing APOs is detection of FGR and timely delivery. Nevertheless, further researches with more cases are required to observe the relationships among the UA velocities, FGR with advanced GA and APOs.
It was showed that the maternal height was significantly and inversely associated with risk of SGA25. A previous reporting suggested that the maternal height was a stronger predictor of birth weight than ethnicity26. Consistent with previous studies25 26, the maternal height of SGA was shortest among the three groups, and was independently predictive of SGA rather than FGR in our study. Conversely, the UA-TAMXV was independently predictive of FGR rather than the maternal height. We speculated that the maternal height was more closely associated with SGA, whereas the UA-TAMXV was more closely related to FGR. Therefore, UA-TAMXV could figure out FGR from SGA when the women were at the same maternal height.
There are some limitations in our study. Firstly, most pregnancies with SGA and FGR were transferred from other hospitals, so it was not always possible to collect actual primary UA Doppler spectrums and serial data. Secondly, as this was a retrospective study, the retrospective design can lead to an inherent risk of selection bias. Lastly, because all of fetuses were delivered at about 37 weeks’ gestation, the pregnancy outcome assessment could not be objectively assessed.
In conclusion, the UA velocities were decreased with the severity of growth restriction and the UA-TAMXV was independently predictive of FGR. The results suggested that the UA-TAMXV might be a new parameter to predict FGR, which might provide a theoretical basis for earlier prediction and better management FGR as well as distinguishing FGR from SGA. Moreover, it might help to determine the optimum time for surgical intervention as well as to select those pregnancies which are in need of close monitoring. However, further studies are needed to confirm these questions.