Strengths and limitations
The strength of the present study is that it is the first large-scale, nationwide, birth cohort study in Japan that investigated various factors contributing to placental abruption in pregnant women. Therefore, the findings of this study can be considered to be representative of the general pregnant population in Japan.24 The prospective data were collected by the physicians, midwives, nurses, and trained research coordinators, and therefore, are more likely to be accurate.
This study also has a few limitations. First, this study lacked the definition of placental abruption along with the data for type (occurred during antepartum or intrapartum) and severity. The severity of placental abruption could have been graded based on the maternal (DIC, hypovolemic shock, renal failure), fetal (non-reassuring fetal status, intrauterine fetal growth restriction, intrauterine fetal death), or neonatal (preterm delivery, small for gestational age, or neonatal death) complications.25 The severity usually causes the premature placental separation. Second, regarding the maternal background data, we relied on a self-reported questionnaire instead of objective measurements of uterine myoma before pregnancy. As such, we were not aware of the size and location of uterine myoma. Third, the specific ART methods (IVF and/or ICSI; cryopreserved, frozen, or blastocyst embryo transfer) were not classified in this study. Finally, although we accounted for several confounding factors based on the questionnaire, unknown risk factors for placental abruption might have existed. Further studies are warranted to elucidate the potential impact of these confounding factors on placental abruption and how these factors can impact the clinical practice of all obstetric care providers.