The first tip from BabyHome on this topic is: avoid making comparisons. Just because your nephew sat when he was four months old that your baby will do the same. And just because your oldest daughter took a while to say her first words doesn’t mean your newborn will take just as long. Each child has their own time.
Some may take their first steps before the time, but take longer to speak. Some might not even crawl until one day they’re stumbling around the house, falling over and laughing as they crawl away throughout the house. Others may laugh or cry more, while some don’t, and so it goes.
The second tip is: always consult your pediatrician if you suspect there’s a delay in your child’s development, such as if the baby, as they grow, doesn’t “harden up”. Or, rather, if you notice the child has some difficulty: with hearing, for instance, if they don’t show curiosity over sounds, even the louder ones. There can be isolated cases and exceptions.
In general, while experts work with developmental milestones – determining the acquisition of certain abilities based on a given age group – there’s no fixed age nor pattern for the baby to develop certain capabilities.
Avoid unnecessary expectations and stimulate your child, without being too pushy. Help them if they struggle with something, shower them with love and incentivize their attitudes and self-esteem. All of this catalyzes learning and a healthy development.