12 week old baby development

Week 12 baby development

by Georgia Aspinall |
Updated on

Congratulations, you now have a three-month-old baby on your hands! We can bet these last 12 weeks have flown by, with your baby slowly developing a little personality and meeting a few new and exciting baby development milestones. This week, watch out for another baby growth spurt and more immunisations. Here you can find out more about week-by-week baby development and what to expect when your baby is 12 weeks old.

12-week-old physical development

At three months old, your baby has grown about an inch per month to a total of three inches since they were born! They’ve also probably gained about three pounds, although it varies from baby to baby.

Like with week 11 of baby's development, you’ll probably be in for another growth spurt this week, which means crankiness, restlessness and feeding more.

As their little body fills out and straightens up, you’ll notice your baby gaining more muscle control and their movements are less jagged. They might even be pushing themselves up when they lie on their tummy. If they need improvement here, try laying down with them as they play and encourage them to look up at you. Tummy time is important, so spend a few minutes each playtime on your tummy with your little one and then build this up to 15-20 minutes.

It’s also a great time to play with wrist rattles and noisy toys, even if your baby didn’t like them a few weeks ago – these toys help your baby’s hand-eye coordination and muscle control. Choose a brightly coloured rattle to engage their vision too, and look for how they react when they make the rattling sound.

12-week-old cognitive development

With their movements becoming more controlled, your baby is starting to understand cause and effect. They’ll start to realise that kicking and punching things, or grabbing at toys, means they move.

Week 12 is a good week to improve your baby’s sense of touch with more skin-to-skin contact. Try baby massage, it also increases bonding and can help their digestive system.

12-week-old baby sleep

Baby sleeping in mum's sling

At 12 weeks, your baby is likely to be sleeping around 12 hours at night broken up by two feeds. Sleep patterns are different for every baby but there are certain things you can expect between three months and twelve months old. Their daytime naps will vary in length, but they will usually have at least three. It’s important for babies to continue to nap during the day; not only will this help your baby sleep well at night but it’s also good for their brain development.

Baby sleep is a problem for many new parents. It can take you a while to understand your baby's sleep patterns and get them into a routine. If you're having ongoing issues with sleep, there are certain tips and tricks you can use to get them snoozing. Try boosting your baby's sleep hormone naturally to help regulate their body clock with the help of sunlight.

How much should a 12-week-old baby be eating?

Now that your baby is becoming an expert in feeding, they will finish much faster, sometimes in as little as 5-10 minutes. For the next few weeks, your baby may start to become distracted during feeds, this will peak at around four months old. While your baby will be sleeping better through the night, they’ll still wake up a couple of times for night feeds.

When it comes to formula-fed babies, they will still need around 150-200ml per kilo of their body weight, although this will decrease over the next few weeks. If you’re breastfeeding, your baby still knows best so follow their lead. Don’t worry too much about how much or how many times you're feeding, as long as your baby is a healthy weight. If you’re worried about your baby’s weight or feeding patterns, you can discuss this with your health visitor or book an appointment with your GP.

How much should a 12-week-old baby be pooing?

Over the past 12 weeks, you’ll probably have seen every kind of nappy imaginable, and the nappy checking doesn’t end yet! When it comes to baby poo, the spectrum of normal is pretty varied – your baby could be pooing after every feed or once every few days. As long as your baby isn’t in pain whilst filling their nappy, all is well. If they do seem uncomfortable, they may be constipated and need to see a doctor.

12-week jabs

It’s that time again, your baby’s second immunisation appointment should happen around this week.

At their first appointment, they received their first dose of the 5-in-1 injection against diphtheria, tetanus, whooping cough, polio and Hib (Haemophilus influenza type B). The 12-week immunisation will give your baby a second dose of this injection, plus another dose of the Rotavirus vaccine (a liquid dropped straight into their mouth). You may be expecting a meningitis C vaccine, but thanks to the success of the immunisation programme here in the UK, this jab was discontinued in July 2016.

12-week-old baby milestones

Settling down

Your baby is finally getting used to being out in the world. They might be developing physically and cognitively every day, but hopefully, your little one will be starting to settle into their routine and understand the world around them. That means crying, restlessness and discomfort tend to settle around now.

Growth spurts

Another one?! Your baby may be starting to settle down more in general, but a baby growth spurt and growing pains could set this back briefly. If you're breastfeeding, they will probably be hungrier and feed more often, but don't worry, everything will go back to normal in a few days.

Cause and effect

As we said above, cause and effect is possibly the biggest milestone your baby will reach this week. You may notice that as they play with their toys and they’ll concentrate harder as the toy moves. That’s your baby realising they can actually make things happen with their little limbs!

Listening to your voice

Something you could start doing this week is reading to your baby. Not only will it relax your baby to hear your voice, but it will also help them understand the rhythm and intonation of language. Try adding story time into the bedtime routine, the sooner you introduce reading the better.


As their little body fills out and straightens up, you’ll notice your baby gaining more muscle control and their movements are less jagged. Tummy time is a great way to help them keep working on this newfound strength as they can practice holding themselves up.

Things to be aware of at 12 weeks

The aptly named fourth trimester is one of the most difficult things to adjust to for any parent. Those first 6 to 12 weeks have likely taken a huge toll on you, so it’s time to celebrate making it through the sleepless nights. Make sure you give you and your partner some alone time, you both deserve it!

Squinting: In the first few weeks of your baby’s life, as their eyes developed and 3D vision improved, you probably noticed a slight squint with all the effort it took to focus. However, at 12 weeks old, if your baby is still squinting, it’s worth raising this with your GP to ensure it’s not a vision problem.

Flat Head Syndrome: If your baby doesn’t like tummy time, or they’re spending a lot of time on their back, you’ll need to look out for a flat spot on the back of their head. This can be managed easily so don't worry too much.

Growing pains: Despite your baby finally settling down, it’s also normal for some babies to become crankier with their three-month growth spurt. All that growing takes its toll and they may become restless and cry more. Just know it’s all normal, and will subside when their growth spurt comes to an end.

Postnatal Depression: Postnatal Depression can occur at any time in the first year of your baby’s life. Are you overwhelmed by guilt or feelings of failure? Do you feel like everything could go wrong and it’s all your fault? You may want to visit your doctor and talk about the possibility of postnatal depression. With one in ten women dealing with it, you’re definitely not alone.

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