Yeap a few of us on here have had similar results. My 19week ultrasound said they could see multiple large placental lakes (also know as venous lakes) which to a degree is completely normal. Everyone (whom is pregnant) has placental lakes. They are the areas where the placenta gets the maternal blood from for exchange of oxygen, waste etc) The placenta has almost like fingers that dip into the lakes of our blood to get its needs for bub. Large ones are a variation of normal. Most people don't have them but a few of us do. These lakes are seen as blood pools between the uterine wall and placenta and when looking at it it looks like swirling stuff. It is a worry but usually completely normal despite some sonographers (especially inexperienced ones) getting into a huff about them. I am told they often get another opinion as it can be a sign of placenta previa.

I had my bub 5weeks ago. Despite the initial concern by myself my pregnancy proceeded as normal and I had a normal delivery with normal postpartum bleeding. When the placenta was delivered they commented on it been huge.