E.K.
I googled around and think they might have been referring to placental lakes. I found this:
Placental lakes are enlarged spaces in the placenta filled with maternal blood. These spaces are also called intervillous spaces because they are found between the placental villi the finger-like projections of the placenta that contain fetal blood vessels . The placental villi float in the intervillous spaces and absorb oxygen and nutrients from the maternal blood.
The blood-filled placental lakes appear nearly black on an ultrasound because they do not reflect sound waves back to the ultrasound machine. Placental lakes can be seen within the placenta or on the fetal surface of the placenta bulging into the amniotic cavity. Slow swirling blood flow (larger arrow) may be seen within the spaces, and the shape of the spaces tends to change with uterine contractions. These features may help to distinguish a placental lakes from a thrombus.
Research has found that placental lakes are present in about two per cent of pregnancies and are more common in thicker placentas. There appears to be no association with placental abruption (bleeding from the placenta during pregnancy), high blood pressure or pre-eclampsia, premature labour, small babies or stillbirths.
Because it would seem placental lakes are so common and have no significance, most sonographers do not report them, or even mention them to women.
Placental lakes are considered to be a normal finding in most cases. However, multiple placental lakes seen early in pregnancy in association with decreased umbilical artery blood flow have been associated with fetal growth restriction. In a patient with placenta previa and previous uterine surgery multiple placental lakes should raise suspicion for placenta accreta or percreta. Multiple placental lakes may also mimic gestational trophoblastic disease.
