Mailbag 37: “What Does the Fox Say” – With Dr. Nathan Fox

This is a transcription of a portion of a Healthful Woman podcast.

Megan’s Question

Our first question today is from Megan in Normal, Illinois. Megan is 25 weeks pregnant with her fifth baby, and her anatomy scan noted a placental lake. She wanted to know how common placental lakes are and what, if anything, this finding means for her pregnancy.

Dr. Fox’s Response

Megan, thank you for writing in, and congratulations on baby number five. A placental lake is an area within the placenta that is filled with the mother’s blood. On ultrasound, it shows up as a dark, black space inside the placenta, which is why it tends to catch people’s attention when they see it on the screen or read it in a report.

The good news is that placental lakes are very common. They’re seen in somewhere up to 20% of ultrasounds. For most women, especially when there are just a few small lakes, they don’t change anything about the pregnancy. Most of the data suggests it’s not a problem, and in that situation, we generally just note it and move on.

Where we start to think a little more carefully is when there are many lakes, or when one or more of them is very large. Some studies have linked that pattern to fetal growth restriction, meaning the baby may not grow as well as expected later in pregnancy. The data here isn’t perfect, and doctors don’t all agree on what to do with it. But in those cases, it’s reasonable to do a follow-up growth scan in the third trimester just to make sure the baby is growing well. That’s an easy, low-risk way to get reassurance.

There is one important distinction I want to make. Not every dark area in the placenta is a lake. Sometimes those dark spaces have a lot of turbulent, high-velocity blood flow when we look at them with Doppler. Those are called lacunae, and they can be associated with placenta accreta, a condition where the placenta grows too deeply into the wall of the uterus. So one has more implications, lacuna, than the other, lake. That’s why it’s important for the ultrasound to be done and read by people who know how to tell the two apart.

So, Megan, if your report simply mentioned a placental lake and nothing else, that’s almost always a normal finding. If there were many lakes or a very large one, ask your doctor whether a growth ultrasound later in pregnancy makes sense for you.

Learn More on the Healthful Woman Podcast

Dr. Fox answers several other listener questions in this episode, including the chances of a successful VBAC after a C-section for arrest of descent, what “cervical swelling” in labor really means, whether a very fast labor is likely to happen again, and whether it’s safe to keep breastfeeding during a new pregnancy. To hear the full conversation, listen to Mailbag 37 on the Healthful Woman podcast, produced by Maternal Fetal Medicine Associates.

If you’re navigating a high-risk pregnancy or have questions about an ultrasound finding like a placental lake, Carnegie Imaging for Women provides specialized maternal-fetal medicine care and support throughout every stage of pregnancy. Contact Carnegie Imaging for Women through the online contact form to schedule an appointment.

Carnegie Imaging for Women blogs are intended for educational purposes only and do not replace certified professional care. Medical conditions vary and change frequently. Please ask your doctor any questions you may have regarding your condition to receive a proper diagnosis or risk analysis. Thank you!