What is a placental lake?

What do placental lakes look like?

Placental lakes show up on ultrasound scans as black areas. They are spaces within the placenta that fill with blood. They are usually small and round or oval in shape (Silver 2023).

This image shows a placenta with several placental lakes. The lakes are above the baby on the front wall of the womb (uterus), and can be seen as three small, black areas on the surface of the placenta.

How common are placental lakes?

They are very common. Although placental lakes sound worrying, placentas often have one or more lakes by the time of the routine 20-week scan (Jones 2021).

Placental lakes are found in about one in five pregnancies by mid-pregnancy (Jones 2021, Schiffer et al 2021). They are more common when the placenta is thicker than usual (Jones 2021).

One or two placental lakes are a normal finding and won't affect your baby (Jones 2021). Most sonographers don't report them or mention them during a scan.

Having placental lakes does not mean that other complications, possibly related to the placenta, are more likely to happen (Schiffer et al 2021). There's no link between having normal-sized placental lakes and:

• bleeding from the placenta (placental abruption)
• high blood pressure
• pre-eclampsia
• premature labour
• babies who have a low birth weight

 

Are placental lakes ever a sign of a problem?

It's unusual for placental lakes to be a sign of a complication (Schiffer et al 2021). You're much more likely to have another sort of problem with your placenta, such as placenta praevia (NHS 2022).

However, there are a few situations when placental lakes may prompt your sonographer to do further checks. These include finding placental lakes early in pregnancy, such as during a dating scan, having lots of placental lakes or if the placental lake is larger than normal (Jones 2021, Schiffer et al 2021). A large placental lake can be up to 5cm across (Schiffer et al 2021).

It's possible that rarer placental lakes like these may be linked to pre-eclampsia or having a baby that is small for your stage of pregnancy (Schiffer et al 2021). But we need more research to be sure.

With these sorts of placental lakes, your sonographer might check the blood flow from the placenta to your baby using a doppler scan. If they have any concerns they will discuss them with you and may refer you to a consultant for further monitoring.

Learn more about the placenta:

• Read more about the placenta: what it is and how it works.
• Learn what happens to the placenta during the third stage of labour.