The first six months are the period where you go from "I have no idea what this little person does" to "I know all their tells". A lot happens. Some of it you'll notice. Some of it you'll miss and then catch on the second time round.
The HSE publishes a list of typical milestones from birth to six months. It's a guide, not a test. The ages given are averages. Most babies hit most of them. A baby who's slightly behind on one and ahead on another is doing fine.
What follows is that list, in plain English, with the things the HSE explicitly flags as worth a conversation with your GP or public health nurse.
By 1 month
Most of what you see in the first month is your baby getting used to being out in the world. Bright objects catch their eye. Loud noises startle them. They cry to tell you what they need.
Motor skills. Lifts their head up briefly during tummy time. Tight newborn fists start to loosen a little.
Communication and hearing. Alert to sounds. Throaty gurgling. Recognises your voice. Prefers high-pitched voices to low ones.
Social and emotional. Getting to know you. Cries to communicate. Crying for 2 to 3 hours across the day is normal.
Problem-solving. Fixes on a bright object and follows it with their eyes. More interested in faces than objects.
By 2 months
This is the month where most parents say "they're a real person now". Babies become more alert, awake for longer stretches, and start smiling back when smiled at.
Motor skills. Lifts their chest during tummy time. Holds their head upright when held. Tracks moving objects past the middle of their body with their eyes. Hands loose half the time. Briefly holds a rattle.
Communication. Looks at people who are talking. Responds to your voice. May start cooing.
Social. Smiles back at you. Enjoys being chatted to and sung to. Often soothed by being picked up.
By 3 months
Cries start to sound different depending on what's wrong. Hungry-cry and bored-cry become recognisable.
Motor. Good head control when seated. Rests on forearms during tummy time. Bats at objects with their hands. Follows objects in a circular pattern with their eyes.
Communication. Chuckles. Echoes back sounds you make ("ooh", "aah").
Social. Warm smiles and laughs in response to you. Soothed by a calm voice. Long eye contact. Excited by rattles.
By 4 months
The head wobble is gone. Most babies have steady head control by this point. Some are already rolling front-to-back.
Motor. Holds head steady in line with body. May roll front-to-back. Sits with back support. Clutches at objects. Reaches and moves hands while lying on their back.
Communication. Laughs out loud. Moves their body towards a voice. Makes "ah-goo" sounds. Vocalises when a person stops talking, as if taking their turn.
Social. Recognises their mother (and usually the main caregiver, by extension).
By 5 months
A quieter month milestone-wise but lots of consolidation. Babies become more responsive to tones of voice.
Motor. Lifts head when sitting.
Communication. Smiles and vocalises at themselves in a mirror. Copies your voice. Makes "raspberry" spitting sounds.
Social. Spontaneous smiles. Reacts to different tones of voice and prefers calm, gentle ones.
Problem-solving. Reaches out and grasps dangling objects. Starts noticing small things.
By 6 months
By six months you have a baby who babbles, sits with support, has clear preferences for the people they know best, and is starting to understand that things they drop don't just vanish.
Motor. Sits with support. Rolls back-to-front. Passes objects from one hand to the other.
Communication. Babbling sounds like "baba" and "gagaga". Turns toward new sounds. Turns toward people entering the room. Enjoys back-and-forth babble conversations with you.
Social. Shows preference for a particular person. May get upset when their main caregiver leaves the room. Recognises familiar faces. Tries to hold their bottle while drinking.
Problem-solving. Looks to the floor when they drop a toy.
The big picture
The HSE lists each milestone with the words "most babies will reach" because most babies do, but some don't on the published schedule and turn out perfectly fine. The diagnostic value of the list is the pattern, not the individual marker.
What matters more than any single month is whether your baby is still moving forward. Skills should be added, not lost. A baby who used to smile and has stopped, or used to coo and has gone quiet, is the situation where the HSE explicitly says to call your PHN.
The full list lives on the HSE developmental milestones page. It's worth bookmarking.
The HSE also publishes guidance for premature babies, who are measured against their corrected age (the age they'd be if born on their due date) rather than their birth age, usually until about two years old.
How parents typically remember all this
Honestly, they don't. The first smile and the first roll get remembered. The fact that the baby started making "ah-goo" sounds at exactly the right week doesn't. Most parents log what they notice and then forget the rest.
That's where having a simple log helps. Not for the baby's sake. For yours, when the PHN asks at the next visit, or when you want to look back at "when did she start sitting" in a year's time and have an actual answer.
Log the firsts as they happen.
Lullagram's Milestones screen lets you log the first smile, the first roll, the first babble in one tap. They appear on the printable healthcare report for your PHN. Free for 7 days.
Get Lullagram on Google Play