You are 17 weeks and 4 days

Your baby's lungs are changing but are still fairly immature and won't be fully developed until week 35.

157 days until your due date

Your baby's lungs are developing in a complex way. Picture them as a tree: the trunk (the windpipe, or trachea) has formed and has branched into medium and smaller branches (bronchi), but the twigs (bronchioles) that hold the leaves (alveoli) haven't formed yet. The alveoli have walls so thin that oxygen can pass from the air inside them into the blood, and carbon dioxide can leave.

From now until 28 weeks, the "twigs" that will hold the alveoli form. They can transfer gases only a little, and the lungs won't work fully until the alveoli develop. The blood vessels supporting the lungs grow alongside them and will be essential for taking in oxygen after birth.

After birth, all the blood leaving the right side of the heart will go to the lungs. But before birth, while the lungs are full of fluid and not used for breathing, only about 10–15 percent of it goes there.

Explainer

Stretch gently

Only stretch as far as is comfortable, because in pregnancy you're more likely to damage muscles and tendons.

Good to know

Stretching safely

Relaxin is one of the most important pregnancy hormones. It relaxes connective tissue, tendons, and ligaments so your diaphragm can expand to make room for your baby, and loosens them enough to help the birth canal open for a vaginal delivery.

Relaxin affects most of your body, making your spine and pelvis less stable, so watch your posture and alignment when exercising:

When standing, keep your hips in a neutral position rather than pushing them to one side.
Avoid arching your lower back and rounding your shoulders.
Keep all movements slow and controlled, and don't stretch beyond your comfort zone. Your extra flexibility could lead to overstretching in activities such as yoga or Pilates.

Relaxin also relaxes the walls of your veins, which can cause varicose veins. Cardio exercise helps boost blood flow and can reduce them.

This is general information based on NHS guidance, not medical advice, and every pregnancy is different. Talk to your midwife or GP about anything that worries you. If your baby's movements slow down or change, or you have bleeding, severe pain or leaking fluid, call your maternity unit straight away. In an emergency, call 999.