Week 18

Days 120 to 126 · 17 weeks 0 days to 17 weeks 6 days old · about 4 months This week many babies measure about 63 cm (24.8 in) and weigh about 6.7 kg (14.8 lb), though babies vary widely and what matters is that your baby keeps following their own curve on the growth chart. Around now, many have roughly doubled their birth weight. Rolling is becoming a real skill, sometimes in both directions, and your baby is learning that their actions make things happen. For you, four months in, life may have found a loose shape, even if the nights are still broken, and your body may be sending new signals, such as a returning period. The big event this week is the 4-month check, with its vaccines, and a chance to ask every question you have been saving up.

Day 120 · 17 weeks 0 days old

Your baby may have doubled their birth weight

Somewhere between about 4 and 6 months, most babies weigh roughly twice what they did at birth. If your baby was born at 3.3 kg (7 lb 4 oz), that means somewhere around 6.6 kg (14 lb 8 oz). It is one of the fastest stretches of growth in a whole human life.

The timing varies a lot. Smaller babies often double sooner, bigger babies later, and breastfed babies tend to gain a little more quickly in the early months and a little more slowly after that. None of this is a test your baby passes or fails on a set date.

The growth chart is the better guide. Your baby's doctor or nurse plots weight, length and head size over time, and the line is what tells the story. A baby who sits steadily on a lower curve is just as healthy as one who sits high. A line that drops across two or more of the main curves, or flattens out, is worth a closer look, and that is exactly what the checkups are for. If your baby was born early, growth is still plotted, but development and feeding are judged by corrected age, so do not compare them with a baby born at term on the same day.

Try this: Find your baby's birth weight and today's weight, and write both on the same page of your notes or phone, side by side.

Day 121 · 17 weeks 1 day old

Getting ready for the 4-month check

In many places, babies have a routine checkup at about 4 months, often on the same day as a round of vaccines. Appointments can feel rushed, and it is easy to forget the very thing you meant to ask. A little preparation helps. Bring your baby's health record or vaccine card, if you have one, and a list of any medicines or vitamins your baby takes. Note roughly how your baby feeds: how often, how long or how much, and how many wet and dirty diapers they have in a day. A rough picture of sleep is useful too. Then think about what your baby is doing. Are they rolling, reaching for toys, laughing, turning towards your voice? Do they hold their head steady when you hold them upright? You will be asked questions like these, and there is no right answer to rehearse. Babies reach these skills over a wide range of weeks.

Most of all, write down your own questions, however small they seem. Questions about poop, rashes, sleep, feeding, or how you are coping are all welcome. If your baby was born early, remind the doctor or nurse of their corrected age.

Try this: Start a list today on your phone titled "4-month check" and add every question that comes to you this week.

Day 122 · 17 weeks 2 days old

Rolling in both directions

You may have seen your baby roll from tummy to back already, often by surprise, with a startled look afterwards. Rolling from back to tummy usually comes a little later, because it takes more strength and coordination: lifting the head, swinging a leg across and pulling the shoulder over.

Many babies roll one way between about 4 and 6 months and both ways by 6 or 7 months, but the range is wide. A few skip rolling for a while and go straight to sitting. What matters is steady progress in moving and using both sides of the body.

The safety steps you read about in week 14 matter even more now: keep a hand on your baby on any raised surface, and use the floor for play. For sleep, still put your baby down on their back every time. Once they can roll both ways by themselves, you do not need to flip them back if they turn over in the night. Keep the sleep space bare, and make sure swaddling has stopped.

Call your baby's doctor if your baby seems floppy or stiff, uses only one side of their body, or has lost a movement skill they used to have. Try this: Lay your baby on their back on a blanket and hold a favourite toy just out of reach to one side, then the other, so they can practise turning towards it.

Day 123 · 17 weeks 3 days old

The 4-month check and vaccines

At the 4-month visit, your baby's doctor or nurse will usually weigh and measure your baby, check head size, and plot all three on the growth chart. They will examine your baby from head to toe, including the heart, eyes, hips and the soft spots on the head, and watch how your baby moves and reacts. They will ask about feeding, sleep, development and how you are doing.

In many places, this visit also brings the second doses of the vaccines first given at about 2 months. These commonly protect against diphtheria, tetanus, whooping cough, polio, Hib and pneumococcal disease, often with hepatitis B and rotavirus. Schedules differ from country to country, so follow your local schedule and ask if you are unsure what your baby is receiving. Staying on schedule protects your baby when these illnesses are most dangerous.

The comfort tips from week 9 still work: feed or cuddle your baby during and after the needles.

Call your baby's doctor if after a vaccine your baby has a fever lasting more than two days, cries inconsolably for more than three hours, or has a large, very red swelling at the injection site. Get help right away if your baby has trouble breathing, swelling of the face or mouth, or becomes limp or very hard to wake.

Try this: Ask the nurse to note today's vaccines in your baby's record, and set a reminder for the next visit before you leave.

Day 124 · 17 weeks 4 days old

Your period may be on its way back

There is no fixed date for the first period after birth. If you are formula feeding, it often returns within about 6 to 12 weeks. If you are breastfeeding, especially exclusively and through the night, it may stay away for many months, sometimes until well after weaning. Combination feeding usually falls somewhere in between.

The first periods can be different from before: heavier, lighter, longer, more crampy or less, or irregular for a while. That is common as your hormones settle. Some people notice a small, temporary dip in milk supply around their period, which usually recovers within a few days with frequent feeds. One fact catches many people out. You usually release an egg about two weeks before your first period, so you can become pregnant before you ever see one. Breastfeeding only offers reliable protection when your baby is under 6 months, feeds only on breast milk day and night, and your periods have not returned. If you do not want another pregnancy yet, revisit the contraception you discussed at your postnatal check.

Call your care provider if you are soaking a pad or tampon every hour for more than two hours, pass clots larger than a golf ball, or have bleeding with fever, strong pain or a bad smell.

Try this: Mark the first day of any bleeding in your calendar this month, so you can start to see your own pattern again.

Day 125 · 17 weeks 5 days old

Your baby is learning cause and effect

Shake a rattle and it makes a sound. Kick the mattress and the mobile jiggles. Smile at a parent and the parent smiles back. Around this age, babies begin to grasp that they can make things happen, and it is a powerful discovery.

You may see your baby repeat an action again and again: batting the same toy, kicking the same spot, dropping something to hear the noise. That repetition is not boredom. It is your baby testing an idea, checking whether it works every time, and storing the result.

You do not need special toys for this. A wooden spoon on a pan, a crinkly cloth, a bracelet of plastic rings or a soft ball with a bell inside are all wonderful. Make sure anything you offer is too big to fit in the mouth whole and has no loose parts, since everything still goes straight to the mouth. People are the best toys of all. When your baby coos and you answer, when they reach and you respond, they learn that their actions matter in the social world too. That simple back and forth is part of what builds trust, language and confidence.

Try this: During awake floor time, hang a light, soft toy where your baby's feet can kick it, or hold one there yourself, and watch them discover that kicking makes it move.

Day 126 · 17 weeks 6 days old

A morning of their own with the baby

By four months, many families have fallen into patterns about who does what. Often the person who gave birth, or who is home more, has become the expert on how the baby likes to be held, fed and settled. That is natural, but it can leave the other parent feeling like a helper rather than a parent. Time alone with a baby changes that. Without someone else stepping in, a partner finds their own ways to soothe, play and solve problems. Babies benefit from close bonds with more than one adult, and those bonds are built in ordinary hours: diapers, walks, naps, a fussy half hour.

For the parent stepping back, it can be surprisingly hard. You may hover, or want to give instructions. Try to let things be done differently. A baby who wears mismatched socks or naps in the stroller instead of the crib is perfectly fine.

If you are a single parent, the same idea applies to a trusted friend or relative. A few regular hours with another loving adult gives your baby a wider circle and gives you real rest.

For your partner: Plan a morning this weekend that is yours alone with the baby, from the first feed or bottle to the first nap, and send the other parent out of the house to do something just for them.