The lead underwriters restrict insiders from selling their shares for a period of time – usually 180 days. However, the underwriters have the option of lifting the lock-up period earlier. Knowledgeable investors track the termination of lock up periods, knowing that stocks may weaken at about the six month mark as more shares become available to the public, weakening the supply/demand dynamics. Investors should be cautious of any IPO that does not have a standard 180-day lock-up.