Approach

Most cases of PP are found on ultrasound ordered for another reason (e.g., dating, fetal anatomic survey, undergoing IVF). Risk factors include uterine scarring (most commonly due to prior cesarean section), advanced maternal age, smoking, previous multiple pregnancies/short interpregnancy intervals or miscarriages/induced abortions, prior PP, infertility treatment, and illicit drug use.

Ultrasound is both sensitive and specific for the diagnosis of PP, although this is operator dependent.[36] It may be performed by a transabdominal, transvaginal, translabial, or transperineal approach, although transabdominal ultrasound is recommended by the American College of Radiology and the American College of Obstetricians and Gynecologists as the initial procedure.[37][38] The Society of Obstetricians and Gynaecologists of Canada, the Royal College of Obstetricians and Gynaecologists, and the Society for Maternal-Fetal Medicine meanwhile recommend transvaginal ultrasound, stating that it is more accurate for the diagnosis of PP than a transabdominal approach, with a sensitivity of 88% and a specificity of 99%.[1][2][5][Figure caption and citation for the preceding image starts]: Placenta previa (previously known as complete previa) at 22 weeksFrom the teaching collection of Janet R. Albers, MD [Citation ends].com.bmj.content.model.Caption@514fd1a6[Figure caption and citation for the preceding image starts]: Placenta previa (previously known as complete previa) at 32 weeksFrom the teaching collection of Janet R. Albers, MD [Citation ends].com.bmj.content.model.Caption@3e815a83

Women with previous cesarean section (or other uterine scarring)

All women with a history of cesarean section should undergo ultrasound to look for placental location.[3] If PP is suspected, referral should be made for color flow Doppler ultrasound to screen for placenta accreta spectrum (in which the placenta is adherent to the underlying myometrium).[2][3][4]​ If placenta accreta spectrum cannot be reliably excluded on ultrasound, MRI of the placenta should be obtained.[2][4][15][39][40][41][42]

Asymptomatic women

If PP or a low-lying placenta are detected at the routine anatomic scan (typically 18 to 22 weeks' gestation), a repeat ultrasound should be performed at approximately 32 weeks to reassess placental location.[1][2][38] In women with a persistent low-lying placenta or PP at 32 weeks, a further ultrasound should be arranged at 36 weeks to aid delivery planning.[1][2]

Women presenting with painless bleeding in late second or third trimester

If a woman presents with second- or third-trimester bleeding and PP has not already been diagnosed, a history should be taken to include known risk factors for PP. These include advanced maternal age, uterine scarring (most commonly due to prior cesarean section), prior PP, and infertility treatments. Other risk factors include previous miscarriage or induced abortion, multiparity/short interpregnancy intervals, and smoking or illicit drug use. With the exception of uterine scarring, these risk factors are neither sensitive nor specific.

Very careful speculum examination may be used to exclude cervical or vaginal hemorrhage as a cause of bleeding. In women in early labor with mild bleeding, this should be done under "double setup" to allow conversion to an immediate cesarean section in case of massive vaginal bleeding. Digital vaginal examination should never be performed before PP is ruled out by other means.[43]

If time permits (e.g., where there is no evidence of fetal compromise or excessive bleeding), urgent ultrasound should be obtained.[37] Urgent consultation with suitably experienced physicians able to perform a cesarean section and manage attendant complications (e.g., cesarean hysterectomy) is recommended.[44][45]

In assessing the hemodynamic effects of bleeding, it is important to remember that most young, otherwise healthy pregnant women tend to have slightly low blood pressure and to be slightly tachycardic. Hemoglobin normally decreases in pregnancy. The value varies with gestational age. It may be as low as 10 mg/dL in mid-pregnancy. Iron deficiency (and, in certain genetic groups, thalassemia) commonly coexists with pregnancy and may result in an even lower hemoglobin.

Type and screen and crossmatch for at least 4 units of packed red blood cells (and inform transfusion service of possibility of the need for massive transfusion). Follow with serial complete blood counts (frequency depends on degree of bleeding). Consider assessing international normalized ratio (INR)/partial thromboplastin time (PTT), fibrinogen, and fibrinogen degradation products if there is any evidence of disseminated intravascular coagulation, such as petechiae, ecchymosis, gangrene, mental disorientation, hypoxia, hypotension, or gastrointestinal bleeding.

A fetomaternal hemorrhage involving a rhesus (Rh)-positive fetus to an Rh-negative mother has significant implications for fetal morbidity in subsequent pregnancies. Women who are Rh negative should have a Kleihauer-Betke test to help determine the degree of hemorrhage, and the need for and amount of Rho(D) immune globulin that is required for prophylaxis for Rh disease in subsequent pregnancies.[5][46]

Alpha-fetoprotein (AFP) testing is usually offered routinely as part of triple or quadruple testing to screen for neural tube defects and other congenital abnormalities. If a second- or third-trimester PP is diagnosed in a woman with an abnormal AFP level, the index of suspicion for invasive placentation should be high and an ultrasound and/or magnetic resonance imaging scan considered.[47]

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