Pregnancy · Your baby

What is your baby actually doing in there?

Hiccups, yawns, thumb-sucking and a surprising amount of practice breathing. A trimester-by-trimester look at the busy little life going on inside your bump.

By Motherhood  ·  7 min read

From the outside, pregnancy can look like a lot of waiting. You get bigger, you get more tired, you get very good at finding the nearest loo. But inside, your baby is having one of the busiest years of their life: building a brain, practising the moves they will need on day one, and getting to know the sound of you.

Here is what is going on in there, stage by stage. Every baby develops at their own pace, so think of the weeks below as a rough guide rather than a timetable.

The first trimester: building the basics

The heart gets going early. By around six weeks, a tiny heart has started to beat, and it is often visible as a flicker on an early scan. It beats much faster than yours, which is why the heartbeat you hear at later appointments sounds like a galloping horse.

They are moving long before you feel it. From about eight weeks your baby starts making small, jerky movements, stretching and twitching as their muscles and nerves connect up. At this point they are around the size of a raspberry, so you will not feel a thing for weeks yet.

The first practice runs begin. By the end of the first trimester, scans can pick up hiccups and early "breathing" movements, where the chest and diaphragm move in rhythm. There is no air in there, of course. This is rehearsal, and it carries on right up until birth.

By your 12-week scan your baby has all their major organs in place, plus fingers, toes and tiny nail beds. From here on, the job is mostly growing and refining.

The second trimester: tasting, swallowing and getting stuck in

They drink, and yes, they wee. From around 12 weeks your baby swallows amniotic fluid and their kidneys start making urine, which goes back into the fluid. It sounds grim, but it is a closed and clever system, and the swallowing helps their digestive system and lungs develop.

Thumbs become a thing. Thumb and finger sucking has been seen on scans from early in the second trimester. It is an early version of the sucking reflex your newborn will rely on for feeding, which is why some babies arrive already very attached to their own hand.

They get a taste of your dinner. Flavours from what you eat pass into the amniotic fluid. A 2022 study led by Durham University used 4D scans to show babies in the third trimester making a "laughter face" after their mums took a carrot capsule and a "cry face" after kale. So if you are eating a varied diet, your baby is already sampling it.

Your baby is not just growing in there. They are rehearsing: breathing, sucking, swallowing and listening, ready for day one.

Fingerprints get their final pattern. The unique ridges on your baby's fingertips form during the second trimester and stay the same for life.

The kicks start. Most people first feel movement somewhere between 16 and 24 weeks. Early on it can feel like bubbles, flutters or a fish flicking its tail. If this is your first baby, it often takes a little longer to recognise what you are feeling. The NHS guidance on your baby's movements is worth bookmarking now.

They are covered in fuzz. Around the middle of pregnancy your baby grows a layer of soft, fine hair called lanugo, plus a waxy coating called vernix that protects their skin from all that time in fluid. Most of the lanugo goes before birth, though some babies arrive with a bit on their shoulders and back, especially if they come early.