I m planning a holiday to pisa in February. How is the weather in February ? Is it too cold for an 18 month baby?

This is a question that nobody can answer for you. If you think about it, there are babies in Pisa as well so it's impossible to say it will be too cold for babies. It's up to you to decide whether you think it's too cold.
Pisa's general weather averages can be found on better weather websites. Take a look there.
Many 18 month old babies live in Pisa and do just fine ! The key is to have the appropriate clothing for the weather. February should be less crowded which makes pushing the stroller much easier.
But - Pisa to me is a day trip, not a destination. Depending on how long a time you have you might consider staying in Lucca. It's more child friendly and (IMHO) much more charming.
I would also recommend Lucca over Pisa as a base.
Thanks a lot. I 've just checked lucca....it really looks like a nice place. Thanks
Hullo, I live nearby Pisa, and I have lived there for so long!
In february, as Pisa is nearby the seaside, winter is going slowly to change in a better weather - even if it is still winter-. It is a very nice time to visit Tuscan cities.
It depends on what the weather is like where you live if you think it is too cold or not. In the winter here in Minnesota we used to take our baby to day care everyday in the car, park it and walk with the baby to the daycare center even when it was -20 degF (-29 degC) outside in the morning every week day. The baby had a one-piece snow suit with mittens and footies and a separate hat. Toddlers (18 months old) had snow suits with separate mittens, hats, shoes and socks.