Put the tip of a pinkie in your baby's mouth and she'll start to suck. Place a finger in her palm and she'll grab it. These reflexive actions aren't things your baby thinks about doing. They're simply her body's way of reacting to particular stimuli.

At their most basic level, these responses (there are about 75 in all) are your newborn's survival instincts at work. Some help her protect herself in this strange new world she's entered. Others aid her in finding food or keeping her body organized, with arms, legs, head, and trunk in the proper alignment for breathing and development; still others have no purpose that we know.

Reflexes are also an indication of a newborn's health. Pediatricians usually check for three or four of these reactions during a newborn's exams  — either in the hospital after birth or during the first office visit. "They tell doctors important things about an infant's nervous system, kind of like a system check tells you your computer is functioning correctly," says Steven Wolf, M.D., director of pediatric epilepsy at Beth Israel Medical Center, in New York City. Reflexes show that messages are making it from body part to brain and back again.

Though it's critical that your baby exhibits these responses, it's also key that she moves beyond them at the right time. Reflexes are controlled by the lower brain stem. As her upper brain develops, it overrides reflexive reactions to allow for more purposeful movement. The fact that she stops responding involuntarily to certain stimuli means that she's moving on to higher thought processes and better physical control. (However, if reflexes don't disappear, it could indicate that parts of your baby's brain aren't developing correctly.)

They can also be an infant's way of communicating. Sometimes a reflex, such as flinching or flailing of arms and legs, is a reaction to something stressful in her environment  — such as a loud noise. "It's a little like she's saying, 'Time out. I need some help here!' This is how she communicates her needs," says Carole Kenner, associate dean of academic advancement at the University of Illinois at Chicago's College of Nursing. That's why understanding the reflexes is a useful skill. "It helps take some of the guesswork out of parenting," says Tiffany McKee-Garrett, M.D., a neonatalogist and an assistant professor of pediatrics at Baylor College of Medicine, in Houston.

Before such reflexes come and go, take note. "Don't just dismiss these signals; see what they mean for your baby," says Kenner. Next, what you need to know about the most essential reflexes. {C}

Reflexes: stepping, rooting & tonic-neck

Walking on Air

You've probably noticed this: If you hold your newborn under his arms, his little legs pump like pistons. But far from displaying amazing athletic ability or showing signs of being an early walker, he's simply exhibiting the walking, or stepping, reflex.

Experts aren't quite sure what's behind this one; some speculate that it might show that babies are programmed from birth to walk, even though they can't actually do it until their muscles are ready to handle the job.

You should catch this reflex early, since it's one of the first to go. It gradually vanishes during the first few months of life. After that, you won't see him take a step for months.

Seeking Sustenance

Gently brush your baby's cheek or mouth with your nipple or finger and she'll turn her head toward it, open her mouth wide, and get ready to suck. That's the rooting reflex. This survival instinct appears right at birth and helps babies find their next meal. "A baby's eyes don't focus well at birth, so this helps her find a nipple when the world is a blur," says Dr. Wolf.

Rooting is especially helpful when you're breastfeeding. Since achieving the proper position and a good latch-on in those first weeks can be a challenge, stimulating this reflex is a great way to get your newborn ready to nurse when you are. By about 4 months, she'll be able to find the breast or bottle on her own and the reflex will disappear.

A Mysterious Position

When your baby is relaxed and lying on his back, gently turn his head toward his right. He'll likely extend his right arm out flat in front of him and bend his left arm up to his head as if he's a pint-size Zorro. (He'll do the same thing if you turn his head to the left.)

This odd response is called the tonic-neck reflex, and experts aren't really certain what the point of it is. It seems to have both benefits and drawbacks for his development. While assuming this position helps him focus on the hand that's extended in front of him, it also prevents him from learning to hold things up near the center of his body, where they'd be easier to play with. Having that arm shot straight out can make rolling over difficult as well. This reflex usually goes away by 3 or 4 months, when your baby is ready to perform both of those tasks. {C}

Reflexes: sucking, grasping, and the Moro

Getting a Mouthful

One of the earliest responses to kick in is the sucking reflex, and it's also among the most important for survival. "Babies actually begin sucking in utero at about eighteen weeks' gestation, though they don't swallow yet," says Dr. McKee-Garrett. Ultrasound pictures have shown them going to town on a finger or other available body part.

Of course, once your baby is born, sucking is the way she'll get all her meals, so it's good that she's got a head start. This reflex is so strong that newborns will try to latch on to just about anything that comes close to their mouth. Put a clean finger in your baby's mouth, nail toward the tongue, and she'll suck away.

This will become voluntary by 2 or 3 months, as babies begin to be more discerning about what's in their mouth and what they're getting from it. That's why breastfed babies may spit out a pacifier or refuse a bottle  — they prefer the real thing. If they still suck on your finger at this point, it's by choice.

A Grip on Life

An especially fun response to elicit is the grasp reflex. Simply stroke your baby's palm with your finger and he'll automatically tighten his grip around it and cling on. "It's an exciting feeling when your newborn grabs you and holds on," says Dr. Wolf. "It's like he really knows you're there." A similar reflex even occurs in babies' feet. Brush the sole of his foot and his toes will curl, as if he'd like to grip your finger with them too.

Parents are often amazed at their child's Herculean grip  — just try and pry him off. Babies can hold on so tightly, in fact, that you can often gently lift them off the surface they're lying on as they cling to your finger. Don't lift him too far though: Since he has little control over his grasp, he can't control when he lets go either. Babies will also grip toys, utensils, and small rattles  — even fistfuls of hair, which can be a painful surprise!

What's this all about? For one thing, it's a way for infants to interact with their environment. They can't consciously hold on to you yet, but touch is such an integral part of their development that nature's found a way to help them do it without even having to think about it. "Babies need lots of skin-to-skin contact," says Kenner. "They need to be held and carried. The warmth and sensation of it gives them a strong sense of comfort."

Most noticeable in the first two months, this reflex usually begins to disappear by the third month and will probably be gone entirely by the time your baby has reached the midyear mark.

Registering Fear

Abrupt movements or loud sounds can elicit a catlike, protective reaction called the Moro, or startle, reflex, says Steven Shelov, M.D., chairman of pediatrics at Maimonides Medical Center, in Brooklyn, New York. When your baby is startled, her arms and legs will move up quickly and extend outward, as if she's saying "Hold me!" with her entire body. If the surprise has been a rather big one and she isn't picked up and cuddled quickly, she may start to cry too.

Doctors often check for this response by sitting an infant up, then abruptly  — but safely  — lowering her back a bit. A newborn who startles easily and often might be comforted by the security of being bundled or swaddled in a light blanket, says Kenner. Some babies have a harder time keeping it all together than others and just feel safer  — and are startled less easily  — when they are held close. The Moro reflex is most prominent in the first month and should vanish by about 4 months.

Reflexes: tongue-thrust, righting and withdrawal

Out of the Mouths of Babes

The tongue-thrust reflex is an important, built-in safety mechanism. It makes sure that all those foreign objects babies are so eager to pop into their mouth don't get far enough inside to become a choking hazard. Instead, your little one's tongue acts like a miniÐsteam shovel, pushing everything out of its way  — even some things you might wish he wouldn't. To try to spoon-feed a baby whose tongue is still thrusting is to fight a losing (and very messy) battle. That's why most doctors suggest that you wait until this reflex goes away before starting him on solids.

Since keeping the wrong items from going down his throat is so crucial, the tongue is backed up by the gag reflex  — just in case. If some large object does happen to get past his tongue and touch the back of his throat, his jaw will lower and he'll gag as he automatically tries to expel the item. The tongue thrust will start to disappear by the middle of the first year, though it can persist until 8 months in some babies. The gag reflex, on the other hand, never goes away.

Getting Comfy

Despite your newborn's inability to move around much, she's not just a blob. She'll spontaneously "fix" herself if she's put down in an awkward position, say, with a leg askew or an arm asunder, to make herself more comfortable, says Dr. Wolf. Known as the righting reflex, this response is also safety related: Place her flat on her stomach, for example, and she'll automatically lift her head and turn it to the side so she can breathe. If a blanket falls over her face, she'll shake her head and flail her arms until she's moved it away.

As your baby gets older and more mobile, other righting reflexes will kick in. When she's just starting to sit up, for instance, she'll automatically put out a hand to keep herself from falling over (that's called "tripoding"). As better muscle tone and control develop, these actions will become more directed and voluntary and less automatic.

A Prudent Retreat

Come at your baby a little too quickly and you'll see him flinch away from you. Similar to the startle reflex, the withdrawal reflex, too, seems to be related to self-protection. "Just like us, babies know when something's coming at them and they want to get out of the way," says Kenner. "They just don't have the experience to know if they'll be hurt or not, so they play it safe."

Infants will also withdraw from pain  — such as when their heel is pricked for a blood test. Though this protective response will get more sophisticated as your little one grows and gains more knowledge of the world around him, it will stay with him  — thankfully  — his whole life.

Mary Arrigo's last article for PARENTING was "Raising a Can-Do Kid," in the August issue.