We collected the data of 4490 pregnancies complicated with PP in two tertiary hospitals from two different provinces, and we observed a high association between adverse pregnancy outcomes and the types of PP. According to the traditional classification method, we found that complete PP and low-lying placenta were the most and least dangerous types, respectively. Moreover, there was not much difference between marginal and partial PP with regard to their effects on maternal and perinatal outcomes (Supplementary Tables 3 and 4). We found that the effects of the four-classification method and three-classification method of PP on maternal and neonatal outcomes were clinically similar (Tables 2 and 3). We also used the two-classification method from AIUM, which combined marginal, partial, and complete PP into PP. We found that PP increases the risk of PAS, PPH, hemorrhagic shock, severe PPH, blood transfusion, hysterectomy, puerperal infection, preterm labor, admission to NICU, and low birth weight (Tables 2 and 3). As per the two-classification method, PP conferred additional risks of marginal and partial PP on adverse pregnancy outcomes.
In line with our study, several studies have suggested that complete PP might be clinically different from incomplete PP, and that the former is associated with the highest risk of worsening maternal and perinatal complications[19,20]. Similar characteristics were observed between women with marginal and partial PP in the study by Dola et al., implying that they might be clinically similar to each other and different from complete PP[21]. However, Daskalakis et al. found that the type of PP did not influence the maternal and neonatal outcomes, except that neonates born to women with incomplete PP had lower Apgar scores than those of neonates born to women with complete PP[22]. Gorodeski and Bahari reported no difference between antepartum, intrapartum, or postpartum bleeding in different types of PP. They also noted that the gestational age, birth weight, and neonatal and perinatal mortality rates were similar among women with all types of PP[23].
In this study, the incidence of low-lying placenta, marginal, partial, and complete PP was 5%, 31%, 4%, and 60% in Third Affiliated Hospital of Guangzhou Medical University and 16%, 24%, 2%, and 58% in Tongji Hospital, respectively. Although the incidence rates of partial PP among the four types were the lowest, the composition ratios of different types of PP were similar in these two tertiary hospitals from two different provinces. The Third Affiliated Hospital of Guangzhou Medical University is the rescue center for major obstetric diseases in Guangdong Province. The proportion of relatively serious type of PP (marginal, partial, and complete PP) is higher than that in Tongji Hospital. In the study by Dola et al., a total of 179 patients had PP, out of whom 37 (21%), 21 (12%), and 117 (67%) patients had marginal, partial, and complete PP, respectively[14]. The relationship between the edge of the placenta and cervical os might change as the gestational age progresses. In our study, the diagnosis of PP was based on the last ultrasound before delivery. We inferred that during late pregnancy, some partial PP might shift to milder type, especially when the cervix begins to dilate.
Initially, PP was distinguished by visual inspection or gentle palpation of the placental edge in a partly dilated cervix during labor[24]. With the application of ultrasound, especially transvaginal ultrasound, PP was diagnosed mainly by ultrasound rather than by palpation. Determining the location of the placenta using ultrasonography during mid-pregnancy was now a routine practice. However, it was technically difficult to differentiate between marginal and partial PP by ultrasound examination, especially as the opposite side of the internal cervical os could not be visualized on ultrasound[10]. Dashe et al.[25] reported an inability to precisely classify partial PP, especially in the absence of complete cervical dilation. Thus, it is debatable whether the traditional classification is currently practical.
In the present study, we found that as per traditional classification, compared with low-lying placenta, both marginal and partial PP increased the risk of PPH, blood transfusion, preterm labor, admission to NICU, and low neonatal birth weight. To further confirm that there was no clinical difference in the adverse pregnancy outcomes of women with marginal and partial PP, we considered women with marginal PP as the reference group (Supplementary Tables 3 and 4). We observed that partial PP did not increase the risk of other adverse pregnancy outcomes, except for increased risks of preterm labor and low neonatal birth weight. Owing to the technical difficulty of ultrasound in distinguishing marginal and partial PP, we proposed the three-classification method, which combines partial and marginal PP into one type, namely “marpartial” PP. As shown in Tables 2, 3, and Supplementary Tables 5 and 6, the three-classification method was clinically similar to the four-classification method in investigating the effect of different types of PP on adverse pregnancy outcomes.
According to the two-classification method, both marginal and partial PP afford risks of PAS, severe PPH, hemorrhagic shock, and hysterectomy, compared with low-lying placenta, however, this is not true (Table 2); only complete PP was the risk factor of those adverse pregnancy outcomes. Adopting this method would lead to wastage of medical resources, and would require more experienced clinicians, more blood, and more women would have to undergo hysterectomy. This could increase anxiety and tension among pregnant women, which is detrimental to good pregnancy outcomes. Hence, the three-classification method is preferred to the two-classification method.
Strengths and limitations
Our study has several strengths. The relatively large sample size, including 4490 singleton pregnancies complicated with PP from two tertiary hospitals from two different provinces, enabled us to estimate the effect of different types of PP on adverse pregnancy outcomes. We first proposed the three-classification method to distinguish PP, and then used multivariate logistic regression analysis in three types of PP. Thus, the three-classification may be practical from the ultrasound and clinical perspective.
Nevertheless, our study has some limitations. This was a retrospective cohort; large and prospective studies in this regard are warranted. Just distinguishing the types of PP was insufficient for accurate prediction of adverse pregnancy outcomes.