A urinary tract infection (UTI) is caused when your urinary system becomes infected by bacteria. UTIs are much more common in women than in men. Half of women will have at least one UTI during their lifetime (CKS 2009).
Cystitis, an infection of the bladder, is a common UTI which you're more likely to develop while you're pregnant (NHS Choices 2009).
If left untreated, UTIs can be quite painful and may cause other problems if the infection reaches your kidneys. If a kidney infection is left untreated during pregnancy your baby could have a low birth weight or be born prematurely (Lloyd 2009, CKS 2009). Your kidneys may also become damaged. So you can see why prompt treatment is important.
You're more prone to UTIs during pregnancy due to the changes your body is experiencing. The hormone progesterone relaxes the muscles of the tubes (ureters) that connect your kidneys to your bladder. This slows down the flow of wee from your kidneys to your bladder. As your uterus (womb) enlarges, it has the same effect. As a result, bacteria have more time to grow before they're flushed out.