Those who have a caesarean section typically have more blood loss than a vaginal birth; so 1000 mL is commonly used to determine excessive blood loss. It is easy to underestimate maternal blood loss because the primary method of assessment is visual observation. Evaluation and diagnosis
Identifying risk factors early in the pregnancy is essential in managing uterine atony and PPH. This allows for planning and organizing the necessary resources including staff, medicines, assistive devices, and the proper blood products. The delivery plan should also be cognizant of the ability of the hospital or facility to provide an appropriate level of care if any complications occur.Most diagnosis of uterine atony are made during the physical exam directly upon completion of the delivery. Diffuse uterine atony is typically diagnosed by patient observation rather than blood loss. The uterus can be directly palpated or observed indirectly using a bimanual examination post delivery. An atonic uterus can feel soft, "boggy" and/or enlarged. Bleeding from the cervical os is also common. If the atony is localized to one area of the uterus, the upper, fundal region may be still be squeezing while the lower uterine segment is non functional. This can be difficult to see with a cursory abdominal examination and easily overlooked. Therefore, a comprehensive vaginal, abdominal, and rectal examination should be performed. The physical examination may include  ultrasound imaging for rapid visualization of the uterus and other causes of bleeding.
Uterine atony is the failure of the uterus to contract adequately following delivery. Contraction of the uterine muscles during labor compresses the blood vessels and slows flow, which helps prevent hemorrhage and facilitates coagulation. Therefore, a lack of uterine muscle contraction can lead to an acute hemorrhage, as the vasculature is not being sufficiently compressed. Uterine atony is the most common cause of postpartum hemorrhage, which is an emergency and potential cause of fatality. Across the globe, postpartum hemorrhage is among the top five causes of maternal death. Recognition of the warning signs of uterine atony in the setting of extensive postpartum bleeding should initiate interventions aimed at regaining stable uterine contraction. Risk factors
There are many risk factors for uterine atony and several are due to the type of labor a mother experiences such as prolonged labor, labor lasting less than 3 hours, uterine inversion, the use of magnesium sulfate infusions, and extended use of oxytocin. Uterine distention caused by things like more than one fetus present, polyhydramnios, fetal macrosomia, uterine fibroids, chorioamnionitis can also lead to decreased uterine function and atony. Retained placental tissue or placental disorders, such as an adherent placenta, placenta previa, and abruption placentae increase the mothers risk of PPH. Body mass index (BMI) above 40 and coagulopathies are known risk factors.Magnesium sulfate is used often in patients with preeclampsia and eclampsia, can inadvertently inhibit uterine contractions. In addition, preeclampsia can lead to blood disorders such as  thrombocytopenia, platelet abnormalities, and disseminated intravascular coagulation.