This article was written by Karen Robock and originally published on Today’s Parent.

If your baby seems to fit that sweet little onesie one day, and have it busting at the seams the next, it may not be your sleep-deprived eyes playing tricks on you. She might be having a major growth spurt—one of many that babies experience in their first 12 months.

Although growth spurts can happen at any time, it’s common for them to hit at about 10 days, between three and six weeks, and several times afterward, often around three months, six months and nine months. And when a spurt strikes, it can be fast and furious; babies can measurably gain weight and length in just 24 hours, says Michelle Lampl, a doctor and growth researcher at Emory University in Atlanta. Her studies show that tots can sprout as much as nine millimetres in length in just one day. Wondering if your babe is in the midst of a spurt? Here’s what you might notice.

1. She’ll sleep like a log (or be up all night)

In the day or so before a big growth spurt, some babies sleep more than usual. “There are important physiological changes that happen during sleep that are essential for growth,” says Peter Nieman, a paediatrician and assistant clinical professor at the University of Calgary medical school. Don’t wake her for feedings unless it’s within the first week and advised by your care provider. “She needs her rest, and she’ll make up for milk or formula she missed at her next feeding,” he says.

2. She’ll be hungry

Some babies prefer food to snoozing during a spurt. Even if she was starting to sleep longer stretches at night, your baby may suddenly want to eat around the clock again.

At about the three-month mark, Carrie Fisher, a Toronto mom of two, noticed a huge spike in her daughter Ramona’s appetite. “I knew she must be going through a growth spurt because she was inconsolable unless she was nursing,” Fisher says. “I thought there might be something wrong with my milk supply because she never seemed full.” If you’rebreastfeeding, don’t worry, your body will keep pace with baby’s appetite. (If you have real concerns that you’re not producing enough milk, talk to your doctor.) Be sure to drink plenty of fluids and recruit your partner or a family member to help with everything that can’t be done with a babe in arms until the spurt passes. If you’re using formula, increase the amount you’re giving very gradually—there is a danger of overfeeding. “If your baby starts spitting up more than usual she’s probably getting too much,” says Nieman.

3. She’ll be cranky

Frequent fussing can be normal for a few days during a growth spurt. “This can seem rather dramatic at the time, but it doesn’t last very long, and babies will calm down after their growth spurt,” says Lampl. Lots of cuddling and reassurance in the meantime will help to soothe her.

This too shall pass

You may feel like that spurt will never end, but it will. Most only last a couple of days. Not convinced that a growth spurt is to blame? Shifts in mood, feeding habits and sleep schedule can also mean that she’s getting sick, starting to teethe (if she’s older than three months) or just needing extra comfort due to a change in routine. Talk to your doctor if you’re concerned that her symptoms might be something more, but don’t stress about exactly how much your baby is growing, or when, and don’t compare her size to that of other babies. At each wellness visit, your doctor will track her growth (measuring length, head circumference and weight). As long as she’s gaining steadily and proportionally there’s no reason to sweat exact ounces or inches, says Neiman. “All babies grow at their own rate and speed—they know what they’re doing.”

A version of this article appeared in our March 2014 issue with the headline “Here she grows,” p. 38.