As a parent, you know your child best. If your child is not meeting the milestones for his or her age, or if you think there could be a problem with the way your child plays, learns, speaks, acts, and moves talk to your child's doctor and share your concerns. Don't wait.

It could be anything from a little-bit-limp muscle tone to a shy personality that keeps your baby sitting tight instead of crawling. Here’s a list of red flags you probably should bring up at your next doctor visit.

The Four-Month Mark

At the end of this newborn era, most babies are much more sociable and responsive than when they first crossed your home’s threshold. Mention it at the four-month checkup if your baby is still not:

• Responding to loud sounds.
• Smiling at not just family members but other people he or she encounters as well.
• Reaching and grasping toys.
• Following objects with his or her eyes.
• Supporting his or her head on her own.
• Babbling.

The Eight-Month Mark

Between the four-month birthday and the end of the seventh month, your baby will change dramatically. The baby will start out quietly sociable but still unable to move much. The next thing you know he’ll be fully upright, boisterous, and able to get across the room in creative ways. Mention it to the doctor if by the eight-month-birthday:

• He still seems to have some of those quirky newborn movements like the Moro or tonic-neck reflex.
• His muscle tone doesn’t seem right—it’s either too stiff or too floppy— and he can’t seem to bear weight on his legs (when you hold him in a standing position with his feet on the floor) or hold his head steady when you pull his body up by the arms to a sitting position.
• He’s not cuddly or affectionate with the people who care for him, and doesn’t show interest in being around people in general.
• Something seems amiss with his eyes: They consistently cross, turn in or out, or tear a lot, or he can’t seem to follow an object with his eyes when you move it in front of his face.
• He does not turn to locate or otherwise respond to sounds.
• He doesn’t actively reach for objects and still can’t get them to his mouth (typically, everything is going into the mouth by the end of this stage).
• He hasn’t reached physical milestones like rolling over or sitting up at the end of this period.
• He still has a lot of difficulty sleeping for any length of time at night.
• He continues to have colic or is frequently inconsolable.
• He does not try to attract your attention through actions or sounds.

The Twelve-Month Mark

Good-bye baby, hello toddler—maybe. By her first birthday, your baby may seem like a little boy or girl. Then again, she may still not have mastered many big-kid skills, as development continues to vary widely. You’ll want to talk to your baby’s doctor if:

• She isn’t attempting much in the way of words. The typical just-turned-one-year-old may say two or three words recognizable only to Mom and Dad, or have a vocabulary of two dozen. But she should at least be babbling a lot.
• She isn’t yet getting around efficiently, be it by crawling or cruising or walking.
• She shows no interest in interactive games like peekaboo or pat-a-cake, and doesn’t search for objects that are hidden while she watches.
• She isn’t pointing to objects or pictures or using gestures like waving or shaking her head no.