Pregnant with 2nd Baby and worried he/she will arrive early like my daughter

Sophie
Hi!
So we have recently found out I am almost 12weeks pregnant and minus the morning sickness and wild mood swings we are so excited!!
I have started to worry about early labour. Our daughter arrived at 34+4 weeks spontaneous labour and never found out why. Thankfully she was only in NICU and SCBU for 11 days before she got to come home and is now a happy, healthy and cheeky 15month old!
I guess what I'm asking is there anyone who has gone into spontaneous labour and never found out why and the same has happened again? People are telling me to stop worrying and getting anxious over it but that's so much easier said than done xxxx