Baby a sitting very low at 27 weeks

Loryn • Mommy to b/g twins born june 20th, 2016 at 31 weeks. Now pregnant with #3! Due Spring 2019!
Had an ultrasound today at 27 weeks, and twin a is really low sitting right on my cervix. Luckily my cervix is a little behind her head but still worries me how low she is. When she moves it feels like she's about to fall out of me! Has this happened to anyone else? How long did you go before having them or are how many weeks are you now? Tia!