Listeria in Pregnancy: Miscarriage, Stillbirth, and Neonatal Listeriosis

  1. Why Pregnancy Increases Listeria Risk 10-Fold
  2. Maternal Infection Symptoms
  3. First and Second Trimester Outcomes
  4. Third Trimester Outcomes
  5. Early-Onset Neonatal Listeriosis
  6. Late-Onset Neonatal Meningitis
  7. Why All Maternal Fever Needs Blood Cultures
  8. Post-Exposure Management
  9. Long-Term Outcomes for Surviving Newborns
  10. Connections

Why Pregnancy Increases Listeria Risk 10-Fold

Pregnant women develop invasive listeriosis at approximately 10 times the rate of healthy non-pregnant adults of the same age. The biological explanation is not simply that "immunity is suppressed during pregnancy" in a general sense — it is far more specific than that.

Listeria monocytogenes is an intracellular pathogen that survives and multiplies inside host cells. Controlling it requires cell-mediated immunity — specifically, the ability of T cells (particularly CD4+ Th1 cells and CD8+ cytotoxic T cells) to recognise and destroy infected host cells. This is the very arm of the immune system that is deliberately down-regulated during pregnancy to prevent the maternal immune system from attacking the fetus (which is immunologically semi-foreign, carrying paternal antigens).

Progesterone is the key driver of this immune shift. Rising progesterone levels from weeks 6–10 of pregnancy onward actively suppress Th1 cell-mediated immunity and shift immune responses toward Th2-type (antibody-mediated) responses. This Th1 suppression is protective for the pregnancy — it prevents miscarriage — but it creates a specific vulnerability to intracellular pathogens that rely on Th1 immunity for clearance, including Listeria, Toxoplasma, and certain herpes viruses.

The placenta itself may also play a role: Listeria expresses InlA (internalin A), which binds to E-cadherin on the surface of trophoblast cells (the placental cells in direct contact with maternal blood). This gives the bacterium direct access to the placental interface, allowing it to invade the fetoplacental unit even when maternal bacteremia is mild or transient.

Maternal Infection Symptoms

One of the most dangerous features of listeria infection in pregnancy is how mild the maternal illness can appear while the fetal danger is severe. Many pregnant women who develop bacteremia describe symptoms they might dismiss as a minor illness at any other time.

Typical maternal symptoms include:

What is strikingly absent: the classic meningismus (stiff neck), photophobia, and severe headache that mark CNS listeriosis. In pregnancy, the bacterium targets the fetoplacental unit far more readily than the maternal nervous system. The mother may feel moderately unwell while the fetus is dying of septicemia and placental infection.

This asymmetry — mild maternal illness, catastrophic fetal outcome — is the central challenge of listeria in pregnancy and the reason that any febrile illness in a pregnant woman who ate high-risk foods must be evaluated urgently.

First and Second Trimester Outcomes

Listeria infection in the first and second trimesters carries a high risk of pregnancy loss. In the first trimester, infection is most likely to cause spontaneous miscarriage, which may not be recognised as listeria-related because miscarriage is common and the maternal illness may have been mild or misattributed.

In the second trimester (weeks 13–26), bacteremia that reaches the placenta can cause:

A study of pregnancy-associated listeriosis cases in the United States found that among cases occurring before 20 weeks gestation, fetal/neonatal death or miscarriage occurred in approximately 70% of cases. Among cases at 20–27 weeks, adverse fetal outcomes occurred in approximately 50%.

The key concept is that the placenta cannot mount an effective immune response — it relies on the maternal immune system for protection, which is already partially suppressed. Once Listeria reaches the placenta, it can multiply in the trophoblast cells and spread to the fetus faster than the maternal immune system can clear the bacteremia.

Third Trimester Outcomes

Listeria infection in the third trimester (weeks 28–40) can result in:

A crucial and distressing pattern in third-trimester cases is that maternal symptoms can be mild or even absent in the 24–48 hours before the fetal outcome becomes apparent. The mother may feel "a bit under the weather" and attribute it to pregnancy fatigue; she may not have significant fever; and then fetal movements decrease, monitoring shows fetal distress, and urgent delivery reveals a critically ill or stillborn infant.

This is why the clinical rule exists: any pregnant woman in the third trimester who develops fever — even a moderate fever of 38°C without obvious source — should have blood cultures drawn before antibiotics are started, and consideration should be given to empirical ampicillin treatment while awaiting results if she has eaten high-risk foods.

With prompt maternal treatment using intravenous ampicillin, the prognosis improves substantially. Studies show that starting treatment within 12–24 hours of maternal symptom onset, before the infection becomes established in the fetoplacental unit, significantly increases the rate of delivering a live, non-septic infant.

Early-Onset Neonatal Listeriosis

Early-onset neonatal listeriosis occurs within the first 24–48 hours of life (most cases within 12 hours) and results from bacteremia acquired during pregnancy — either transplacentally from maternal bacteremia or from ingesting infected amniotic fluid during delivery.

The clinical presentation is dramatic and often immediately life-threatening. Neonates may present with:

Mortality from early-onset neonatal listeriosis is approximately 25–50% even with immediate treatment in a neonatal intensive care unit. Prematurity worsens the prognosis — a 28-week premature infant with granulomatosis infantiseptica faces a very different survival prospect from a 38-week neonate with the same infection. Treatment is intravenous ampicillin plus gentamicin for 14–21 days, with duration extended if CNS involvement is documented.

Late-Onset Neonatal Meningitis

Late-onset neonatal listeriosis presents between 7 and 28 days after birth (median approximately 14 days). Unlike early-onset disease, which reflects infection acquired during pregnancy, late-onset disease is thought to result from environmental exposure — either from the birth canal during delivery, from human contact (nosocomial transmission has been documented in nurseries), or occasionally from breast milk.

The clinical picture is dominated by meningitis rather than septicemia. Neonates present with:

Compared to early-onset disease, late-onset neonatal listeriosis has a better prognosis. Mortality is approximately 10–15%, and a higher proportion of infants survive without permanent neurological injury. The reason is partly that late-onset disease occurs in infants who are somewhat older and stronger, but also that late-onset meningitis is more amenable to effective antibiotic penetration into the CSF without the overwhelming multi-organ sepsis of granulomatosis infantiseptica.

CSF culture confirms the diagnosis; the organism grows readily in culture. Treatment is intravenous ampicillin plus gentamicin for a minimum of 21 days for meningitis.

Why All Maternal Fever in the Third Trimester Needs Blood Cultures

The clinical rule that any fever in a pregnant woman warrants blood cultures — regardless of how the patient appears or what symptoms accompany the fever — is directly derived from the biology of listeria in pregnancy.

The key points that underlie this rule:

  1. Maternal symptoms are unreliable: A pregnant woman can be bacteremic with listeria and feel only mildly unwell. There is no symptom combination that reliably distinguishes listeria bacteremia from viral illness, urinary tract infection, or other common causes of fever in pregnancy.
  2. The fetus does not have symptoms: Decreased fetal movements may be the only signal that the fetus is in danger — by which time placental infection may already be advanced.
  3. Treatment is highly effective if early: Intravenous ampicillin clears listeria bacteremia rapidly. A mother who receives ampicillin before placental invasion is documented has an excellent chance of delivering a healthy infant. Once the fetus is bacteremic, outcomes become much less certain.
  4. The test is simple and low-risk: Drawing blood cultures costs almost nothing and poses no risk to the pregnancy. There is no reason not to draw cultures on any pregnant woman with unexplained fever.

Current guidance from the CDC and infectious disease societies recommends that any pregnant woman with a febrile illness (temperature at or above 38°C / 100.4°F) who has consumed high-risk foods in the past 70 days should have blood cultures drawn, and that empirical treatment with ampicillin should be strongly considered — particularly in the third trimester — without waiting for culture results if the index of suspicion is high. The risk of a short course of ampicillin to the pregnancy is minimal; the risk of delayed treatment to the fetus is potentially catastrophic.

Post-Exposure Management of Exposed Pregnant Women

When a food recall linked to Listeria contamination occurs, or when a pregnant woman reports eating a high-risk food and asks her provider what to do, the guidance depends on whether she has symptoms.

If symptomatic (fever at or above 38°C):

If asymptomatic but known to have eaten recalled product:

Women who call a midwife or obstetrician after seeing a listeria food recall on the news deserve a real answer, not reassurance. The risk from a single exposure to commercially recalled food is genuinely low — but it is not zero, and for a pregnant woman in the third trimester, the consequences of even a low-probability infection are severe enough that the monitoring protocol is always appropriate.

Long-Term Outcomes for Surviving Newborns

Neonates who survive listeriosis — particularly early-onset disease and late-onset meningitis — face a significant risk of long-term neurological sequelae. The severity of these outcomes correlates with gestational age at delivery, severity of the acute infection, and degree of CNS involvement.

Published follow-up studies of neonatal listeriosis survivors document the following outcomes:

Early-onset disease (granulomatosis infantiseptica) that does not include CNS infection — that is, bacteremia with skin and organ lesions but not meningitis — has a better long-term neurological prognosis than late-onset meningitis, assuming the infant survives the acute septic phase.

Following discharge, all surviving neonates should receive audiological assessment at 3–4 months, developmental assessment at 12 and 24 months, and ophthalmological review. Parents should be counselled that neurological outcomes are often not apparent until the child reaches developmental milestones in the first 1–2 years of life.

Connections

Key Research Papers

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  9. Silk BJ et al. Invasive listeriosis in the Foodborne Diseases Active Surveillance Network (FoodNet), 2004–2009. Clin Infect Dis. 2012;54 Suppl 5:S396–404. — Search PubMed
  10. Schlech WF 3rd. Epidemiology and clinical manifestations of Listeria monocytogenes infection. Microbiol Spectr. 2019;7(3). — Search PubMed
  11. Craig AM et al. Listeria monocytogenes during pregnancy: case series and review. J Maternal Fetal Neonatal Med. 2019;32(6):1014–1021. — Search PubMed

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