Momentum is a good servant but poor master. There's no point being in on the ground floor unless you're still standing at the end. That's why your investment decisions should always be rational and based on some knowledge. It's hard enough to get it right even then. Certainly follow momentum, but only if the underlying story makes sense.
Many companies come to market prematurely.
Their early share price price performance owes more to post flotation hype and restricted selling than intrinsic prospects. Pre-flotation investing can be lucrative if you're smart. Earlier stage is not necessarily less certain. Many companies go public far too early. If you invest in IPOs, you often pay for substance that has not yet been delivered.