Trimester-by-Trimester: What Changes, What to Modify

Your guide to moving safely and confidently through pregnancy – trimester by trimester.

Pregnancy (and motherhood!) is one long, wonderful lesson in adaptation. Your body is changing week by week, and so is the way you should move it. If you’ve ever finished a workout wondering “was that actually okay for me to do?”, you are not alone, and you’re in exactly the right place.

At Centred Mums, we support women through pregnancy exercise every single day, whether that’s in our prenatal Pilates classes in St Albans or through our online programmes. Here’s a friendly, trimester-by-trimester guide to what typically changes in your body and how to modify your movement to match it.

First Trimester (Weeks 1–12): Listen More Than You Push

The first trimester is often the most deceptive one, because on the outside very little has changed – but on the inside, everything has. Hormones like relaxin and progesterone are already beginning to rise, energy levels can dip dramatically, and nausea or fatigue may make even your usual routine feel like a big ask.

What’s changing:

• Fatigue and morning sickness are common and very real
• Your abdominals are working hard to protect a rapidly developing baby
• Blood volume is increasing, which can affect stamina

What to modify:

• Give yourself permission to slow down or take rest days – this is not the time to chase personal bests
• Avoid overheating; keep workouts at a comfortable intensity and stay hydrated
• Start paying attention to your breath and pelvic floor connection now, so it becomes second nature later
• If you’re new to exercise, prenatal Pilates is a gentle, safe way to begin building strength from the ground up
• If you’re already exercising you can continue with your usual routine (unless it involves overheating in something like hot yoga) but listen to your body – if you feel like you need to take a step back then do!

Second Trimester (Weeks 13–27): The Sweet Spot — With a Few New Considerations

Many women feel their best during the second trimester – nausea eases, energy returns, and the bump is visible but not yet restrictive. It’s a great window to build strength and flexibility, but your body is quietly shifting in ways that matter for how you move.

What’s changing:

• Your growing bump shifts your centre of gravity, affecting balance and posture
•  Relaxin continues to loosen ligaments around your joints and pelvis
• The abdominal wall begins to stretch, and some women notice the first signs of abdominal separation (diastasis recti)

What to modify:

• From around 16 weeks, avoid lying flat on your back for extended periods – try semi-reclined or side-lying positions instead
• Steer clear of exercises that create excessive doming or bulging through the midline; this is where pregnancy-specific Pilates really earns its keep, teaching you how to work your core without straining it
• Be cautious with deep twisting or high-impact movements as your joints become more mobile
• Focus on pelvic floor and deep abdominal connection – this is the foundation for a smoother birth and postnatal recovery

Third Trimester (Weeks 28–40+): Preparing the Body for Birth

As your due date approaches, movement becomes less about building and more about preparing, releasing tension, and staying comfortable.

What’s changing:

• Your bump is now significantly affecting your posture, balance and breathing capacity
• The pelvic floor is under increasing pressure as baby grows and descends
• Ligament laxity is at its highest, and many women feel achy hips, backs and pelvises

What to modify:

• Prioritise pelvic floor relaxation as much as strengthening – an overly tense pelvic floor can make your birth more difficult
• Slow down; wide, controlled movements are more suitable than fast or ballistic ones
• Use props and modifications generously – this is exactly what a specialist prenatal Pilates teacher can support you with
• Keep breathing at the centre of every session; it connects you to your body and calms your nervous system ahead of birth

The Golden Rule Through Every Trimester

Whatever stage you’re at, the same principle holds true: **your body is not the same body you exercised in last month, so don’t expect it to move the same way.** Pregnancy exercise isn’t about pushing through, it’s about tuning in.

This is exactly why generic workouts don’t always serve pregnant bodies well, and why pregnancy-specific movement matters so much. At Centred Mums, every class is designed around what your body actually needs – supporting your pelvic floor, protecting your abdominal muscles, and helping you feel strong, capable and connected right through to birth and beyond.

Ready to move through your pregnancy with confidence? Explore our Pregnancy Pilates classes in St Albans or get in touch to find the right fit for you — nothing is TMI.