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Pregnancy normally lasts between 37 and 42 weeks. Having a baby before the 37th week of pregnancy is called a premature birth.

Sometimes doctors start labor early on purpose because the baby isn't growing well, or because the mother has a serious medical problem such as very high blood pressure (a condition called preeclampsia). But most babies are born early because:¹

• The mother goes in to labor before 37 weeks (called preterm labor)
• The mother's water breaks before 37 weeks and before labor starts (called premature rupture of membranes).

It's hard to say who will have a premature birth.

Most early births happen without any clear reason.

But we do know that women from poorer backgrounds have a higher risk of giving birth prematurely than other women.²

You're also more likely to have your baby early if:

• You have given birth early before³
• You have had a miscarriage late in pregnancy
• You are carrying more than one baby (half of all sets of twins and most sets of triplets are born early)
• You smoke⁴
• You have an infection in your birth canal or uterus⁵
• Your cervix is weak (called cervical incompetence) and tends to open (dilate) early
• Your placenta bleeds (called antepartum hemorrhage)
• You are African-American.⁶

This information is for educational use only, and is not a substitute for prompt professional medical advice. Readers should always consult a physician or other professional for advice and treatment. ©BMJ Publishing Group Limited 2008. All rights reserved.