Why moving in pregnancy makes your baby stronger

Movement shapes a baby's body, even when they aren't yet born

How do you make an unborn baby exercise?

I was about 36 weeks pregnant with my first baby. We’d gone into London for a friend’s birthday afternoon tea. I’d squeezed my globular body into the largest dress I owned, had put on the hottest flat shoes I owned (very unhot) and had waddled from bus to train to tube. Fortunately it was winter, so I’d swathed myself in my favourite large coat and flattered myself that I was the same person I’d always been. 

But I wasn’t entirely the same. My centre of gravity had gone for a walk. Aided, no doubt, by generous quantities of scones and cream, it was now somewhere far in front of my spine. And the sophisticated set of nerve and muscle reflexes that coordinate posture and balance had been sadly left behind. As a result, I took a step down the stairs, leaning further back than usual to compensate for the extra weight. I didn’t have enough grip on my shoes to withstand the extra shear force. My feet shot out in front of me and I tobogganed down the seemingly endless flight of brass-edged stairs at Victoria tube station on my rump.

There’s a stereotype that people on the London Underground exist only inside their own little bubbles. I can tell you that if a woman bounces legs first down the stairs, her coat riding up as if deliberately to display her enormous belly, then EVERYONE stops. There were gasps, I think someone screamed, and there were SO MANY concerned faces. People offered me a blanket, an ambulance, a place to sit down, a hand to stand up, and a cup of tea to make everything better. 

What a lovely story of people coming together to help someone in need.

The memory still makes me cringe. I should have just taken the lift. 

The thing is, while stair sliding isn't great during pregnancy, taking the lift may actually have been worse for my baby....

Babies start to move well before their mothers can feel them - often before eight weeks gestation. The first movements to occur are fast whole-body movements called ‘startles’, which are soon followed by individual limb and finger movements, and then hiccups and practice breaths.

This 12-week fetus already moves its eyes, stretches, and yawns.

At about twelve weeks, the baby starts to stretch and yawn. They suck and swallow the amniotic fluid that surrounds them. They start to move their eyes. All of these movements stimulate growth of important tissues including the palate, jaw, and retinas.

After twenty weeks, the baby manipulates, squeezes, and occasionally sucks on the umbilical cord. Their posture and position changes often, especially in the first half of pregnancy. At this time, barely touching the sides of the uterus, the baby is less affected by the mother's activity. But once it's reliably touching at least one side, mother's movement moves baby - when pregnant women bend, twist, stretch, and carry, their trunk muscles, ribs, pelvis, and diaphragm squeeze the baby in various ways. After months of only making spontaneous movements, the baby now has a chance to respond reflexively. Withdrawing from those gentle impacts strengthens the reflex loops and starts teaching the brain how to respond to potential threats. Motor learning has begun.

The baby has most space earliest in pregnancy, and as they grow, their movements become more affected by the mother’s movements.

I learned this fact firsthand during my second pregnancy. Baby Bradbury 2 was a fetus when his sister, now Toddler Bradbury 1, contracted some ungodly virus and vomited twenty times in one night. Every time I leaped up to try to upend TB1 in the washing-up bowl, I could feel BB2 doing somersaults in my belly. The mother’s movements move the baby. The washing-machine effect when combined with the vomiting was powerfully nauseating.

Whether or not you're activating them with nocturnal puke-crunches, fetuses have reflexes long before they're born. This set of very early, or 'primitive', reflexes allows the baby to react to the environment in ways that increase its chance of surviving its birth and first few months of life. They help the baby to breastfeed, roll over, grasp, and eventually crawl and walk. As the baby does more and more deliberate movement, the simple primitive reflexes are overridden by postural reactions, which are then overridden by active voluntary movements.

(NB. Interestingly, the primitive reflexes are not deleted. They're still there, but hidden (or "integrated"), and they can be visible again even in adulthood if you have certain head traumas).

Primitive reflexes don't only drive motor function and tissue development, they also drive sensory function. In engineering, to calibrate a sensor, you need to expose it to a known stimulus and measure the response. In this biological system, the known movements caused by primitive reflexes are the known stimulus, and that's how we start developing our senses of sight, balance, and knowing-what-angle-our-joints-are-at-without-looking (or ‘proprioception’). These are the senses we most rely on to move about. And moving about is how we get more input to better train those senses. This is a (sort of) exponential system, right? 

We can see the effect of disrupting the system when movement is restricted. There are a small number of babies who aren’t able to move in the womb. Some babies physically can’t move themselves - babies with nerve or muscle abnormalities may hardly move at all. Their joints become fused and their bones thin and weak. Less extreme, some babies can’t move because their legs don’t have room - babies growing in a ‘breech’ position (i.e. head up instead of head down) have their legs in a narrower part of the uterus, resulting in more hip abnormalities, thinner bones, and altered walking development.

The other way to restrict a baby's movement in the womb is to not give it movements to reflexively react to. Babies with mothers who spend a lot of time sitting still before and during pregnancy are denied motor and sensory input from the mother's movements. They are born with less mature brains, and are slower to develop movement and cognition.

Hang on a moment. If I don’t move around much in my pregnancy, my baby may be impaired? That’s not something that I heard in pregnancy at all. I was told about moving around to make my birth easier or to reduce some of those pesky pregnancy side effects - but not to make my baby better adapted for life.

What were you told about movement in pregnancy? How did your pregnancy affect how you moved?

So what can we do?

• Change the messaging around exercise and movement for pregnant people. Let’s move away from reassurance, “mild exercise won’t hurt you or your baby” towards motivation, “the more you move, the more your baby develops their body and brain.

• Help pregnant women to see that they can build movement into their day in lots of ways. Sitting on the floor, for example, uses much more energy to get down, move position, and get up than does sitting in a chair.

• Provide free exercise classes for all pregnant women, prescribed by midwives. Social prescribing is becoming increasingly common to prevent and treat diseases - let’s roll that out to protect the next generation too.

• And whatever we do in pregnancy, let’s make sure that our young babies have plenty of movement opportunities once they are born. We often strap our babies into devices (pushchairs/strollers, car seats, high chairs) that stop them doing the movements they need to do, often for good safety reasons - but what movements CAN we allow them to do? What restraint is necessary for safety, and what’s just because our culture doesn’t recognise that babies need to move?

Our culture tells pregnant women that it’s much safer to just sit still, but the science tells us otherwise. My compromise? Hold the handrail when walking on brass-edged stairs.

Notes

Primitive reflexes: https://www.ncbi.nlm.nih.gov/books/NBK554606/

Breech babies are more likely to have weaker bones in adolescence: https://link.springer.com/article/10.1007/s00198-019-04945-4

Breech babies have toddler hips that move differently: https://www.sciencedirect.com/science/article/abs/pii/S0378378207000850

Movement during pregnancy affects a baby’s brain maturity: https://www.tandfonline.com/doi/abs/10.1080/13803395.2016.1227427

Movement during pregnancy affects a baby’s neuromotor development: https://pubmed.ncbi.nlm.nih.gov/30817721/

Movement during pregnancy affects a baby’s cognitive development: https://pmc.ncbi.nlm.nih.gov/articles/PMC10107927/