Your baby is making breathing movements and even hiccupping inside your womb. Later in pregnancy you'll be able to feel these hiccups. As there is no air inside the womb, the baby is breathing in amniotic fluid rather than oxygen.
By now most of your pre-pregnancy clothes will be feeling rather tight. When shopping for maternity clothes, you'll need to look for fabrics such as cotton that are comfortable and keep you cool. If you're planning to breastfeed, try to find clothes that might also be suitable for nursing, such as front-fastening blouses. You might also need to buy shoes in a larger size, as many women find their feet swell during pregnancy. You may feel more comfortable in flatter styles because your bump will make it harder to balance.
Most women experience some degree of backache during pregnancy at 18 weeks, as their breasts and bumps become larger. Try not to slouch or slump your shoulders or tip your abdomen too far forward. Make sure you bend your knees when lifting, or better still, get someone else to pick up heavy objects. Dads can help by gently massaging your shoulders. If you're having treatment for back pain, such as chiropractic, make sure you tell the person treating you that you are pregnant. If you have sudden or severe pain in your back or abdomen, consult your GP urgently.