https://en.m.wikipedia.org/wiki/Implantation_(embryology)#Implantation_failure
Reproduction in humans is not very efficient. Only around 30% of natural conceptions result in successful pregnancies. Of the failed pregnancies around 85% are due to implantation failure.
Seems like it's extremely normal for a fertilized egg to fail to implant and be flushed out during your next period.
Usually they are barred from selling immediately following an IPO because it looks really bad and can trigger a price drop with everyone selling all at once
https://www.investopedia.com/ask/answer/12/ipo-lockup-period.asp