How pregnancy changes your body for exercise

How does pregnancy change your heart rate and breathing during exercise?

Your heart works harder in pregnancy, so the same pace can feel more demanding than it did before. Resting heart rate rises by about 15–20 bpm over non-pregnant values¹. At the same moderate effort late in pregnancy, your heart rate may be about 8 bpm higher than before pregnancy⁴.

Breathlessness is common too, especially in late pregnancy, both at rest and after effort¹. During steady exercise at a comfortable level, though, breathing tends to feel less of an effort¹.

A simple check works well. Moderate activity raises your heart rate, makes you breathe faster and feel warmer, and you can talk but not sing². If you can't hold a conversation, slow down³. Brisk walking, water aerobics and stationary cycling at moderate effort are all examples⁵.

How hard an effort feels doesn't track heart rate well in pregnancy, so you may be working at a much higher heart rate than it seems¹. If you use a heart rate monitor, combining it with how hard the effort feels may be the best way to keep track of intensity⁴. Let the pace come down rather than pushing through.

If breathlessness or dizziness doesn't settle once you stop, get medical attention; the emergency signs are listed at the end of this page. If it is mild and passes, still mention it to your midwife or doctor.

Why do your balance and joints feel different in pregnancy?

Pregnancy hormones such as relaxin make your ligaments softer and stretchier, and as your baby grows your posture and centre of gravity begin to change^6,7. Relaxin together with changes in posture can lead to back pain⁸. As you gain weight, your tummy and pelvic floor muscles stretch and your balance shifts forward, so the joints of your lower back and pelvis work harder⁶. How much the hormones make joints unstable isn't clear yet, so don't assume your joints are loose or unreliable¹.

Balance is affected after the first trimester, and pregnant women are 2–3 times more likely to be injured by falling than non-pregnant women¹. Walking changes as well: steps become slower and shorter, and your feet sit further apart, which seems to help keep you steady from side to side⁹. Weight through your feet also moves from the heel towards the midfoot and forefoot⁹.

Pelvic girdle pain is common, reportedly affecting 20% of pregnant women¹⁰. Staying active may help: exercise in pregnancy reduces how severe low back and pelvic pain are, although it doesn't appear to reduce how often they happen¹¹.

What to do:

• Take your time when getting up, turning or changing direction.
• Choose activities with little risk of falling, on steady, even surfaces.
• Keep up gentle, regular activity.
• Mention new back or pelvic pain to your midwife, doctor or physiotherapist.

Which positions become uncomfortable or unsafe as your bump grows?

The main one to watch is lying flat on your back, especially after about 16 weeks. The weight of your bump can press on the main blood vessel bringing blood back to your heart, which could cause low blood pressure and make you feel dizzy³. Resting on your back has been linked with symptoms of low blood pressure in up to 10% of pregnant women¹². Standing motionless for long periods can have a similar effect in some women¹.

Most national guidelines advise against exercising on your back after 16 weeks, though that advice rests mainly on expert opinion¹². How much risk it carries isn't clear yet, so the cautious choice is to avoid it.

What to do instead:

• Skip sit-ups and similar exercises that keep you on your back for more than a few minutes³.
• Prop yourself up with pillows or lie on your side. Pelvic girdle pain exercises are safe throughout pregnancy, but positions may need adapting as your bump grows¹³.
• Try hands and knees for gentle movements, with knees under hips and hands under shoulders¹⁴.
• Get up slowly. You may feel faint after lying on your back or standing up quickly, and it's important to sleep on your side after 28 weeks⁸.
• On days when it all feels like too much, do what you can and return to your usual routine when you're able⁵.

If lying on your back makes you feel faint or dizzy, change position and get up slowly. Dizziness that keeps coming back should be checked by your midwife or doctor.

When to check with a health professional

If you're starting exercise after a long break, it's always a good idea to see your doctor first.

For an injury or a health condition, see a health professional, such as your doctor or a physio.

If you feel uncomfortable or sore in a muscle or joint, stop or ease off, and get advice from your midwife, doctor or physiotherapist if it persists or gets worse.

In pregnancy, bleeding from the vagina, fluid leaking, regular painful contractions, the baby moving less than usual, pain or swelling in a calf, or a bad headache or changes in vision need your midwife, doctor or maternity unit straight away, even if they seem mild.

Call your local emergency number for chest pain or pressure (or pain spreading to the arm, neck, jaw or back, especially with sweating or feeling sick), severe breathlessness, fainting or collapse, signs of a stroke, a serious injury, or someone who isn't safe. In pregnancy, heavy bleeding is also an emergency.

If you have thoughts of self-harm, contact a local crisis line (findahelpline.com lists free services in every country), or call your local emergency number if you are in danger.