The end of the fourth trimester
Twelve weeks: second vaccinations in the UK, and the unofficial close of the 'fourth trimester'. The frantic newborn phase is genuinely behind you — a more interactive, more predictable stage is opening up.
The 12-week vaccinations (UK)
This week UK babies are offered the second doses of the 6-in-1 injection, MenB injection and rotavirus drops — the same trio as at 8 weeks. (Since the July 2025 schedule change, the pneumococcal vaccine now comes at 16 weeks instead.)
As before: paracetamol after the MenB dose as advised by the nurse, comfort feeding freely, and a possibly grumpy, feverish 24–48 hours that then passes.
In the US there's no 12-week appointment — the second vaccine round lands at the 4-month well-visit — so consider this a UK-labelled week with an American breather.
What twelve weeks looks like
Typical (not obligatory) twelve-week form: steady-ish head control, strong push-ups in tummy time, smooth tracking of moving things, easy smiles, rich cooing, hands busy grabbing at everything within range.
Crying is usually a fraction of its 6-week peak, and colic — for the families who've been in its trenches — most often resolves between 3 and 5 months. If you're still waiting, you're near the exit.
Night sleep frequently shows one decent long stretch now, though babies remain magnificently inconsistent — and a wobble around the 3-month spurt is standard.
From surviving to living
The 'fourth trimester' framing holds that babies spend their first three months finishing the womb work — needing constant holding, feeding and soothing. That phase is now closing: your baby can be awake, content and interested without being held every second.
What comes next is more fun and differently tiring: play, laughing (soon), rolling, and opinions. The ratio of delight to endurance keeps improving from here.
Nothing changes on safe sleep yet: back to sleep, clear cot, same room as you until at least six months. When your baby starts rolling both ways, keep placing them on their back and let them settle as they choose.
And you
You have done twelve weeks of the hardest shift in family life. Whatever mixture of methods, milks and muddling-through got you here — it worked, and you're allowed to be proud of it.
Keep a weather eye on your own mind even as things ease: postnatal depression can surface at any point in the first year, in any parent, and treatment works at every stage. Feeling worse as things get 'easier' is a flag worth raising, not hiding.
And a small forward look: the next stretch brings the 3-month growth spurt, first laughs, rolling, and — eventually — the 16-week vaccinations (UK) or 4-month visit (US). Same boat, calmer waters.
Feeding at this stage
Pick how you're feeding — we'll remember for next time. Every one of these is a good way to feed a baby.
Breastfeeding
- Twelve-week feeding is typically fast, efficient and distractible — shorter feeds with good nappies and growth mean skill, not shortage.
- The 3-month spurt may land any day now — a feeding-heavy 48 hours is the fix, not a failure.
- However long you breastfeed — weeks, months or years — every stretch has counted; if you're weighing up what's next, your health visitor or a breastfeeding counsellor can help you plan without pressure either way.
- Vitamin D drops continue daily for breastfed babies throughout the first year.
Breast + expressed
- If you've been exclusively expressing for twelve weeks, that is a serious logistical achievement — and supply is usually stable enough now to survive streamlining a session if you need your life back.
- Comfort feeds after this week's vaccinations work by bottle too — closeness plus sucking is the medicine.
- Freezer stash rotation matters as it grows: oldest milk first, and check your storage times against current guidance.
Breast + formula
- Combination feeding at twelve weeks is a settled, sustainable pattern for many families right through the first year — no expiry date on it.
- Offer whichever feed soothes fastest after the jabs; a feverish baby wants comfort, not consistency.
- If the 3-month spurt arrives, decide again where the extra hunger goes — breast, bottle or both — knowing supply will follow whatever you choose.
Formula
- A made-up bottle for straight after the 12-week injections is the same good trick as last month.
- Feeds are probably well-established now — appetite will step up with the 3-month spurt, so let your baby order the bigger portion.
- First infant formula remains the only milk needed until 12 months, when full-fat cows' milk can take over — nothing to change or upgrade between now and then.
Totally normal (even when it doesn't feel it)
- Fever and grumpiness in the 48 hours after the 12-week vaccinations — expected, especially after the MenB dose; paracetamol as advised and extra cuddles.
- A sleep wobble or feeding frenzy around now — the 3-month growth spurt is the last of the trio (3 weeks, 6 weeks, 3 months) in this guide's window.
- No rolling yet — plenty of babies don't roll until 4, 5 or 6 months.
- No laugh yet either — giggles most often start around 3 to 4 months.
- Still waking at night for feeds — entirely normal at three months and for a good while yet, whatever the internet's champion sleepers claim.
- A baby who's dropped their epic newborn sleep totals in favour of nosiness — more awake time is development, not insomnia.
- Mixed feelings as the newborn phase ends — grief and relief in the same breath is the standard parental condition.
Worth checking
You know your baby best — if any of these ring true, or something just feels off, it's always OK to ask.
- Fever of 38°C (100.4°F) or above not within 48 hours of this week's vaccinations — your baby is under 3 months for a few more days, so keep treating it as an automatic same-day call: GP or NHS 111 (UK); in the US, your pediatrician.
- Post-vaccine fever above 39°C, lasting beyond 48 hours, or with a baby who is drowsy, pale or hard to rouse — call your GP or 111; in the US, your pediatrician. If unresponsive: 999 (UK) or 911 (US).
- Blood in the nappy, a swollen tummy or episodes of severe crying with legs drawn up in the days after the rotavirus dose — call your GP or 111 urgently; in the US, your pediatrician or the ER.
- A baby not smiling, not making sounds, not fixing on faces, or with very poor head control by the end of the third month — book the GP or health visitor for a proper look; in the US, your pediatrician. Early checks help, whatever they find.
- Any non-blanching rash, seizure, blue lips or breathing struggle — call 999 (UK) or 911 (US).
- Fewer wet nappies, dry lips and unusual sleepiness — call your GP or 111 today; in the US, your pediatrician.
- Still feeling flat, anxious, hopeless or detached now the 'hard part' is meant to be over — please call your GP or health visitor this week; in the US, your doctor or 988. Twelve weeks of coping is not evidence you should keep just coping.