Fluid? Help please

Melissa Carrie🐠 • Addictions PHD, Mommy by day (2 kids...11 pregnancies) Don`t dish out what you can`t take!
I had spotting (very light) at 7w5d so I went in for and ultrasound. Today I got a call that there was a very small sac of fluid in the gestational sac which could leak out or the body might absorb. She wasn't concerned sounding at all so I'm not too worried but was hoping someone might be able to explain what causes this? I was measuring ahead and the baby had a hb of 155☺️ Thanks!