No safety issue if it is just a regular sleep sack (not swaddle) that fits well. My DD could do anything in her sleep sack, including walking. She just stopped wearing one in January when she moved to a twin bed.
As long as it's not a swaddle and their arms are free, no issue.
Sept 2013 started TTC#2 (never got PP period so hoping for the first egg to drop)
Jan 2014 - diagnosed POF (Premature Ovarian Failure) and told pregnancy not possible without egg donor because my ovaries would never function again with my hormone levels (FSH 136, LH 98.6, AMH <.03, estradiol 0, HCG 7) - using acupuncture, chiropractic care and TCM herbs / supplements
March 11, 2014 ovulated despite every doctors prognosis!!!
Oct 2014 hormones (FSH 48, Estradiol 89, HCG 1) not good, but heckuva lot better!
Nov. 16, 2015 FET of single adopted 5AA embryo - BFN
Not sure if its related to the sleep sack but DD just started rolling and would cry middle of the night because she seemed to be having trouble. Last night we put her in footsie pajamas and no middle of the night crying. She likes to sleep on her side and we could see on the video monitor she was struggling. Again, not sure but seemed like it may have been the sack.
No safety issues. My boys both slept in Halo sleepsacks for 2 years. And they didn't have any problems moving around or standing up. Definitely much safer than sleeping with a loose blanket, plus it can't be kicked off during the middle of the night.
Re: Rolling over in a sleep sack
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It's safe. My DD's head was at the opposite end of her crib the other night when I got her up in the morning. That is a whole lot of wiggling!
DS1: Quinn - 10.22.10 and DS2: Cole - 01.18.13