**5. Placenta variants**

*marginal praevia* or praevia maginalis (the edge of the placenta extends to the edge of the cervical os), and *low-lying placenta* defined as within 2 cm of the cervical os, without covering it [2]. The reported incidence of the condition is 1 in 200–250 pregnancies [1]. Among the risk factors, there are prior Caesarean delivery, previous abortion, prior intrauterine surgery, smoking, multifetal gestation, increase in parity, and increased maternal age. The risk for placenta praevia is 12 times higher in women with history of placenta praevia in a previous pregnancy. Some studies demonstrated an increased rate of placental insufficiency in women with placenta praevia [18]. However, in a retrospective study of women with a complete or partial praevia, no fetal growth restriction was diagnosed [19]. The placenta location must be recorded during the ultrasound scan in the first- and early second-trimester pregnancies. If the placenta is significantly low, an additional ultrasound scan at the beginning of the third trimester allows the final diagnosis. Patients should be aware that nothing can be done to prevent placenta praevia. The appropriate delivery in placenta praevia is by Cesarean section, as dilation of the cervix causes separation of the placenta, leading to bleeding from the opened vessels. Still, in cases of a low-lying placenta, as the bleeding morbidity has proven to be limited, a vaginal delivery remains an option [1]. Every hospital must have a suitable protocol or algorithm for the management of placenta praevia, as

**Figure 3.** Ultrasound image of complete placenta praevia percreta in a patient with a previous Cesarean section (color

Doppler examination showing the penetration of the placenta into the bladder).

330 Congenital Anomalies - From the Embryo to the Neonate

this is a condition with high maternal and fetal morbidity and mortality [20].

Vasa praevia is a rare condition, in which the fetal blood vessels traverse the lower uterine segment in advance of the presenting part, unsupported by either the umbilical cord or placental tissue (e.g., **Figure 4**). This pathologic structure can cause fetal blood loss, with significant neonatal morbidity or death in case of spontaneous rupture of membranes or amniotomy. Also, fetal heart decelerations and bradycardia can occur if compression of these vessels appears, due to the presenting part [20]. This condition is encountered in 1:2500–5000 pregnancies [21]. The prenatal diagnosis is made with a high accuracy by ultrasound, with a sensitivity of 100% and a specificity

**4. Vasa praevia**
