May 7, 2010 — Labor induction, a multiple pregnancy, and cesarean delivery each increases the risk of rare but deadly amniotic fluid emboli, UK researchers report.

Older ethnic-minority women are also at higher risk, according to the article in the May Obstetrics & Gynecology.

„Induction of labor was associated with a population-attributable risk of 35% in our study, suggesting that, assuming causality, if induction of labor were no longer performed, 35% of cases of amniotic fluid embolism could be prevented,“ said lead author Dr. Marian Knight of the University of Oxford and colleagues.

Of course, they add, labor induction „clearly will continue, and amniotic fluid embolism remains a very rare complication“ — but one to keep in mind when considering the risks and benefits of induction.

Using national UK Obstetric Surveillance System data from 2005 to 2009, the researchers found 60 confirmed cases among an estimate of more than 3 million maternities (for estimated incidence of 2.0 cases per 100,000 births).

They note that this rate, based on prospectively collected data, is lower than reported in retrospective studies from Canada and the U.S. (6.1 and 7.7 cases per 100,000 births, respectively).

After adjustment, amniotic-fluid embolism was significantly associated with induction of labor (odds ratio, 3.86) and multiple pregnancy (odds ratio 10.9). This risk was also higher in older, ethnic-minority women (odds ratio 9.85).

Regarding what she called „a possible increased risk of dying from amniotic fluid embolism amongst ethnic minority women,“ Dr. Knight speculated in email to Reuters Health that „reasons for this…may be related to underlying additional medical problems or access to care.“

Cesarean delivery was associated with postnatal amniotic-fluid embolism (odds ratio 8.84).

The emboli occurred at a median gestation of 39 weeks, within a 6-hour range around delivery. All of the women had at least one cardinal sign of an embolism (shortness of breath, hypotension, hemorrhage, coagulopathy, and premonitory symptoms), and more than a quarter of them had at least four signs.

Fetal membranes ruptured at or before presentation in 92% of cases.

Twelve women died, giving a case fatality rate of 20%. These women were significantly more likely to be from ethnic-minority groups (odds ratio, 11.8).

Seven women received exchange transfusion or plasma exchange. All seven of these women survived, although there were too few of them to be able to infer that this treatment is more effective than other approaches. „These therapies should be regarded as an extension of supportive care and not as a substitute,“ the investigators said.

Outcomes were known for 37 neonates born to mothers with amniotic fluid embolism before or during delivery. Five of these babies died. The perinatal mortality rate was 135 per 1,000 total births.

„Occurrence of amniotic fluid embolism does appear to be associated with induction of labor and caesarean delivery and it is important therefore that both risks and benefits of labor induction and cesarean delivery are considered by clinicians on an individual basis for all women,“ Dr. Knight said.

„We have no indication from this study that the occurrence has become more frequent in the UK,“ she said.

Obstet Gynecol. 2010;115:910-917. Abstract

Reuters Health Information 2010. © 2010 Reuters Ltd.

Clinical Context


Amniotic fluid embolism is a leading cause of maternal mortality. Prospective surveillance has been conducted in the United Kingdom through the monthly mailing of the UK Obstetric Surveillance System, with the aim of estimating the current incidence of the condition, risk factors predisposing to the condition, case fatality, and outcomes.

The previously recorded rates of amniotic fluid embolism were 6.1 per 100,000 births in Canada and 7.7 per 100,000 in the United States.

Study Highlights


  • This is a prospective, population-based cohort and nested case-control study using data from the UK Obstetric Surveillance System collected between 2005 and 2009.
  • System case notification cards were sent to clinicians at all hospitals in the United Kingdom with a maternity unit asking if women with amniotic fluid embolism had been diagnosed.
  • Nil reports were sought to monitor card return rates and use the denominator for calculating incidence.
  • When a case was reported, details of presentation, management, and outcomes were obtained, and all data were collected anonymously.
  • Maternal deaths from amniotic fluid embolism were reported through the Centre for Maternal and Child Enquiries.
  • Potential factors underlying amniotic fluid embolism were assessed with use of logistic regression.
  • All 299 eligible UK hospitals contributed data with 100% participation, and data collection was complete for 97% of cases.
  • There were 60 confirmed cases in an estimated 3,049,100 maternity cases, with an estimated incidence of 2.0 cases per 100,000 maternities.
  • There was no significant change in incidence during the 4-year study.
  • Women with amniotic fluid embolism were likely to be older and to be of minority ethnicity if older than 35 years.
  • The odds ratio was 2.59 for women older than 35 years vs those younger than 35 years.
  • The odds ratio for ethnic minority women older than 35 years was 9.85.
  • The proportions of minorities were 47% Asian, 31% black, 6% mixed race, and 5% Chinese.
  • Amniotic fluid embolism was significantly associated with induction of labor (adjusted odds ratio, 3.86) and multiple pregnancy (adjusted odds ratio, 10.9).
  • The population-proportional attributable risks are 35% for induction of labor, 13% for ethnic minority women 35 years or older, and 7% for multiple pregnancy.
  • 26 of 60 women had amniotic fluid embolism after delivery, and 19 (73%) had it after cesarean delivery.
  • The adjusted odds ratio for cesarean delivery was 8.84.
  • 33 women (56%) had signs or symptoms of embolism at or before delivery, and the remaining had signs or symptoms after delivery.
  • The median gestational age at presentation of the embolism was 39 weeks, and most women presented clinically within 6 hours of delivery.
  • All women had at least 1 cardinal feature of shortness of breath, hypotension, maternal hemorrhage, coagulopathy, or premonitory symptoms, and 16 (27%) had 4 or more of these features.
  • Fetal distress occurred before maternal collapse in 19 cases (32%).
  • 55 (92%) of women had ruptured membranes at or before the time of presentation, and amniotic fluid embolism occurred a median of 45 minutes after rupture of membranes.
  • An obstetrician was present at the time of the event in 28 (47%) of cases and an anesthetist in 27 (45%); neither was present in 26 (43%) of cases.
  • 28 women (85%) who presented at or before delivery underwent cesarean delivery.
  • 12 women died, and the case fatality rate was 20%. All of the women died within 1 day of the amniotic fluid embolism at a median of 1 hour, 40 minutes after the acute event.
  • Deaths were more likely to occur among ethnic minority groups (odds ratio, 4.64), and this association persisted after adjustment (odds ratio, 11.8).
  • 45 (94%) of the 48 women who survived were admitted to the intensive care unit, and the median length of stay was 3 days.
  • The perinatal mortality rate was 135 per 1000 total births.
  • In 3 cases, neither mother nor infant survived.
  • The authors concluded that the rate of amniotic fluid embolism was lower than previously recorded and that risk factors included induction of labor, cesarean delivery, multiple births, and minority status.


Clinical Implications


  • Risk factors for amniotic fluid embolism include induction of labor, cesarean delivery, multiple births, and minority status.
  • Case fatality for amniotic fluid embolism is 2.0 per 100,000 births, and the rate is 20%, with a perinatal mortality rate of 135 per 1000 total births.