Most new parents closely monitor their infants to ensure they're hitting those ever-important milestones. But to be honest, some milestones are more significant than others. Which really matter and which can parents let slide? Is missing these targets anything parents should worry about?

"When I was a new parent, I was so incredibly hung up on if my child was meeting these milestones within the 'normal' or 'accepted' timeframe," says Leslie Herman, a mother of three in Van Meter, Iowa. "Quite honestly, I was fanatic about every first event – first solid food, first pair of fancy shoes, first music class, first time in swimsuit – it was ridiculous."

And if her children didn't meet a developmental deadline, Herman admits she would talk with other parents or family members to get their opinions on whether that was OK. And she's not alone. Milestone meltdowns are an all-too-common parental worry.

So which targeted behaviors really need to fall within the recommended guidelines? Four milestones that Dr. Jennifer Shu, a pediatrician in Atlanta and co-author of Heading Home with Your Newborn: From Birth to Reality (AAP, 2005), has her parents look for are social smile (around 2 months); sitting (about 6 months); walking (about 13 months); and first word (around 1 year).

Brenda Nixon, author of The Birth to Five Book: Confident Childrearing Right from the Start (Revell, 2009), agrees, saying two major milestones in the first year are motor development – which include the ability to sit without support around 6 months and mobility in crawling, cruising and eventual walking by or before 14 months – and attachment.

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