Preterm Labour

What Increases Your Risk

It is hard to predict who is at risk for preterm labour. Some women with risk factors do not have early labour. Others with no known risk factors do have early labour.

Of all women who see a health professional about their preterm contractions (before 37 weeks, without preterm premature rupture of membranes (PROM), about half will actually deliver at full term.⁴

Preterm labour and preterm birth

Most premature births happen after naturally occurring, or spontaneous, preterm labour (as opposed to a medically necessary preterm birth, when the baby must be delivered as quickly as possible to prevent harm to mother or baby).

Experts say that spontaneous preterm labour is often the result of a combination of factors. The most common medical risk factors for a spontaneous preterm birth, in order from most to least risk, are:⁵

• Pregnancy with twins, triplets, or more. (Use of assisted reproductive technology (ART) or superovulation increases the risk of multiple pregnancy, which carries a high risk of premature birth and resulting medical complications.⁶)
• In vitro fertilization (IVF), a type of ART. IVF twins tend to be born earlier than naturally-conceived twins.⁶
• A past preterm delivery.
• Vaginal bleeding in the second trimester.
• Infection in the urinary or reproductive tract, including the vagina.
• Age younger than 18 years.
• Mother's low body weight for height (body mass index).
• Cigarette smoking during pregnancy.
• Frequent contractions.

Other factors that increase your risk for premature labour include:

• Use of cocaine or methamphetamine.
• Too much amniotic fluid (polyhydramnios).
• A significantly shortened cervix, as seen on ultrasound.

Less common conditions linked to spontaneous preterm labour include:

• A cervix that doesn't stay tightly closed until near your due date (incompetent cervix).
• A history of two or more induced abortions. Risk increases with each additional abortion.⁷
• A new pregnancy within 3 months of the end of your last pregnancy.
• Problems with the uterus, such as an abnormally-shaped or overstretched uterus (too much amniotic fluid or twins or more can overstretch the uterus and rupture the amniotic sac).
• Previous surgery on your cervix, such as a cone biopsy. Having a loop electrosurgical excision procedure (LEEP) also raises preterm labour risk.⁸
• Exposure to the drug DES (diethylstilbestrol) before your own birth.
• Low pre-pregnancy weight or low weight gain during pregnancy.
• Not eating a balanced diet.

Medically necessary preterm birth

About 1 in 4 premature births are medically necessary. They are started (induced) early because of dangerous pregnancy complications. These are called indicated preterm births.⁵ The most common medical risk factors for an indicated preterm birth, from most to least, are:⁵

• Pre-eclampsia.
• Fetal distress.
• Poor fetal growth (intrauterine growth restriction, or IUGR) that endangers the fetus's healthy survival.
• Placenta abruptio, which is early separation of the placenta from the uterine wall.

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Author:	Kathe Gallagher, MSW Carrie Henley	Last Updated: April 10, 2007
Medical Review:	Joy Melnikow, MD, MPH - Family Medicine Anne C. Poinier, MD - Internal Medicine Gregory A L Davies, MD, FRCSC, FACOG - Maternal-Fetal Medicine William Gilbert, MD - Perinatology
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