Causes and Risk Factors - a quick look

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Although the cause of preterm birth is often unknown, expectant parents need not leave it purely to chance. There are a number of things a potential or expectant mother should and should NOT do, to limit the chances of preterm birth.


Healthy mother, healthy baby

The cause/s of preterm birth may be due to a number of very different events or triggers. Although there are a lot of risks identified there are no good predictors of preterm birth. This section is intended to give you a guide and a better understanding of the risks, some of which are easier to avoid than others depending on your personal circumstances.
Preemie help is here to help you identify any factors you may be able to avoid.


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Placenta Previa & Placental Abruption

There are many different factors that have been associated with preterm delivery. Sometimes medical problems for the mother can result in a preterm birth. These can include problems with the placenta, such as placenta previa and placental abruption. These are 2 common reasons that can cause bleeding during pregnancy and can cause harm to mother and baby. They are common reasons for elective, preterm delivery.

Placenta previa & Preterm Labor

Placenta previa occurs when the placenta lies abnormally low in the uterus such that it is close to or covering the cervical opening. Placenta previa can often result in preterm labour and is the leading cause of vaginal bleeding. Placenta previa usually occurs during the 2nd or 3rd trimesters. Mothers with placenta previa are at risk of preterm labor, and although they used to be treated in hospital it is considered safe to treat expectant mothers as an outpatient if the baby is less than 30 weeks gestation and neither baby nor mother are in distress.

Risk Factors for Placenta Previa

Risk factors for placenta previa include;

  • sociodemographic status (e.g. age, gravidity, education, marital status, race)
  • behavioral factors (prenatal care, smoking, alcohol use)
  • previous preterm delivery
  • medical and obstetric factors (e.g. caesarean delivery, previous previa)


Research has confirmed that women with a placenta previa compared with controls (i.e. women without a previa) is significantly associated with preterm delivery before;

  • 28 weeks gestation (3.5% versus 1.3%)
  • 32 weeks gestation (11.7% versus 2.5%), and
  • 34 weeks gestation (16.1% versus 3.0%)

A study which examined the relationship between placenta previa and fetal growth restriction in a cohort of singleton live births in New Jersey, USA, reported that the association between low birth weight and placenta previa is primarily due to preterm delivery rather than due to fetal growth restriction. The authors conclude that the risk of fetail growth restriction is slightly higher for women with a previa but is of little clinical significance.

In relation to placenta previa and twins, another study found that the rate of placenta previa was 40% higher among twin births than among singleton births. The risk factors for a previa were similar between singleton and twin births.

(Ananth, Demissie, Smulian, & Vintzileos, 2001; Ananth, Demissie, Smulian, & amp Vintzileos, 2003; Zlatnik, Cheng, Norton, Thiet, & Caughey, 2007)

Placental Abruption

Placental abruption is a serious condition in which there is separation of the placenta from the uterine wall before delivery, it is life-threatening to mother and baby. The separation limits the flow of blood, oxygen, and nutrients to baby and can cause severe bleeding to mother and baby.

A placenta abruption also increases the risk that your baby will be born preterm, will have growth restriction, or be stillborn. When significant placental abruption happens the baby must be delivered as soon as possible, which often results in a preterm delivery.

In most cases a placental abruption results in vaginal bleeding but sometimes the blood stays in the uterus behind the placenta. Woman with an abruption usually experience abdominal pain and rigidity. If you have any signs of a placental abruption you should go to the hospital for a complete evaluation.

Placental Abruption & Preterm Delivery

Your doctor will decide how serious the placental abruption and bleeding is to help determine if the baby needs to be delivered immediately. If your baby is very premature and you only have a minor abruption then your baby’s birth may be delayed, as long as it is not dangerous for you or your baby. At this stage your obstetrician will weigh the risks of a worsening abruption with the risk of a premature delivery. Expectant mothers with a high risk of a preterm delivery may be given corticosteroids to help develop the lungs of your baby.

Risk Factors for Placental Abruption

It is not always clear what has causes a placental abruption but the most common reasons include;

  • a previous placental abruption
  • chronic hypertension, gestational hypertension, or preeclampsia
  • too much amniotic fluid
  • early bleeding in the pregnancy
  • carrying multiples
  • smoking tobacco or cocaine use


Pregnancies complicated by abruption result in increased frequency of perinatal death, decreased fetal size and gestational age. A large hospital-based cohort study in the United States reported that preterm birth proportions among women with an abruption were 39.6% compared with 9.1% of women without an abruption.

(Ananth, Berkowitz, Savitz, & Lapinski, 1999; Ananth & VanderWeele, 2011; Tikkanen, 2011)

Technical Reference List

Ananth, C. V., Berkowitz, G. S., Savitz, D. A., & Lapinski, R. H. (1999). Placental abruption and adverse perinatal outcomes. JAMA : the journal of the American Medical Association, 282(17), 1646-1651.
Ananth, C. V., Demissie, K., Smulian, J. C., & Vintzileos, A. M. (2001). Relationship among placenta previa, fetal growth restriction, and preterm delivery: a population-based study. Obstetrics and gynecology, 98(2), 299-306.
Ananth, C. V., Demissie, K., Smulian, J. C., & Vintzileos, A. M. (2003). Placenta previa in singleton and twin births in the United States, 1989 through 1998: a comparison of risk factor profiles and associated conditions. American journal of obstetrics and gynecology, 188(1), 275-281.
Ananth, C. V., & VanderWeele, T. J. (2011). Placental abruption and perinatal mortality with preterm delivery as a mediator: disentangling direct and indirect effects. American journal of epidemiology, 174(1), 99-108.
Tikkanen, M. (2011). Placental abruption: epidemiology, risk factors and consequences. Acta obstetricia et gynecologica Scandinavica, 90(2), 140-149.
Zlatnik, M. G., Cheng, Y. W., Norton, M. E., Thiet, M.-P., & Caughey, A. B. (2007). Placenta previa and the risk of preterm delivery. The journal of maternal-fetal & neonatal medicine : the official journal of the European Association of Perinatal Medicine, the Federation of Asia and Oceania Perinatal Societies, the International Society of Perinatal Obstetricians, 20(10), 719-723.



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