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A comprehensive review published on April 9, 2026, in Frontiers in Medicine synthesizes decades of evidence on the perinatal transmission of Borrelia burgdorferi (Bb), the bacterium responsible for Lyme disease.

Titled “Perinatal transmission of Borrelia burgdorferi: advancing scientific and clinical understanding of Lyme disease in pregnancy,” the paper stems from a June 2022 Banbury Conference at Cold Spring Harbor Laboratory. It brings together international experts to address a long-understudied public health concern: the ability of the Lyme spirochete to cross the placenta and potentially cause adverse outcomes in fetuses and newborns.

Although Lyme disease is the most common vector-borne illness in the United States and Europe, research on its effects during pregnancy largely stalled after 2000.

This new review highlights that Bb has been known since the early 1980s to cross the placental barrier, yet the true incidence of congenital infection, the full spectrum of symptoms in exposed children, and optimal prevention strategies remain poorly defined.

Strong Evidence of Mother-to-Child Transmission

The authors document clear biological evidence that Bb can infect the fetus:

• Bb has been cultured or detected by PCR in fetal tissues, placenta, cord blood, and organs (including heart, brain, kidney, liver, and spleen) in cases of miscarriage, stillbirth, and neonatal death.
• In one study of asymptomatic pregnant women in endemic areas, 5% of placentas tested positive for Bb DNA.
• Among 62 reviewed human cases of suspected congenital infection, 30% (19 cases) involved mothers with no documented history of Lyme disease and no symptoms during pregnancy—suggesting silent maternal infection can still transmit to the fetus.
• Transmission via breast milk remains unconfirmed in humans but is biologically plausible; Bb DNA has been detected in the breast milk of untreated infected women.

Animal studies reinforce the findings, showing transmission rates ranging from 10.2% in mice to as high as 73% in certain species, depending on strain, dose, and gestational timing.

Clinical Manifestations in Infants and Children

There is no single, well-defined “congenital Lyme syndrome,” but the review catalogs a wide range of potential outcomes in gestationally exposed infants:

• Perinatal complications: Preterm birth, intrauterine growth restriction, hyperbilirubinemia, respiratory distress, suspected sepsis, and placental inflammation.
• Congenital anomalies: Cardiac and genitourinary malformations, hydrocephalus, ophthalmologic issues, and musculoskeletal problems.
• Neurologic and developmental effects: Hypotonia, developmental delay, spasticity, polyneuropathy, autonomic dysfunction, sensory/motor deficits, and emotional lability.
• Severe outcomes: Stillbirth, neonatal death, and later-emerging issues such as relapsing annular skin rashes.

Importantly, some exposed children appear healthy at birth, while others experience severe morbidity—highlighting the unpredictable nature of the infection.

Adding to the problem, standard two-tier serologic testing (ELISA + Western blot) is unreliable during pregnancy. Maternal serology can be negative even when direct evidence (PCR, culture, or immunohistochemistry) confirms Bb in placental or fetal tissue. The authors emphasize that clinical diagnosis should take precedence over serology, and call for broader use of direct-detection methods.

The authors argue that prompt antibiotic treatment of infected pregnant women significantly improves outcomes: adverse pregnancy results occurred in only 11% of treated pregnancies versus 50% of untreated ones. However, Bb has still been detected in placental and fetal tissues even after standard antibiotic courses.

They conclude that perinatal Lyme transmission is a genuine public health issue supported by biological, clinical, and epidemiologic evidence, yet “fundamental questions remain unanswered.” It's associated with a wide spectrum of adverse pregnancy outcomes, ranging from subtle developmental issues to fetal loss. The authors stress that closing these knowledge gaps is essential as Lyme disease incidence continues to rise globally.

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