Mother-to-child transmission of syphilis

Syphilis is one of the most common sexually transmitted infections globally, with approximately 6 million new cases each year. If a pregnant woman who is infected does not receive early and effective treatment, she can then transmit the infection to her unborn infant. This is known as congenital syphilis.

Mother-to-child transmission of syphilis, or congenital syphilis, is usually devastating to the fetus if maternal infection is not detected and treated sufficiently early in the pregnancy. Most untreated primary and secondary syphilis infections in pregnancy result in severe adverse pregnancy outcomes.

Latent (asymptomatic) syphilis infections in pregnancy also cause serious adverse pregnancy outcomes in more than half of cases. The fetus can easily be cured with treatment, and the risk of adverse outcomes to the fetus is minimal if the mother receives adequate treatment with benzathine penicillin during early pregnancy – ideally before the second trimester.

The burden of morbidity and mortality due to congenital syphilis is high. In 2022, WHO estimated 700 000 congenital syphilis cases and 390 000 adverse birth outcomes globally. These adverse birth outcomes included:

• 150 000 early fetal deaths and stillbirths
• 70 000 neonatal deaths
• 55 000 preterm or low-birth weight births
• 115 000 infants with a clinical diagnosis of congenital syphilis.

Of these adverse birth outcomes, 21% occurred in pregnant women who did not attend antenatal care; 53% in women who attended antenatal care but were not screened for syphilis; 16% in women who tested positive for syphilis but were not treated or received inadequate treatment; and 9% in women who tested positive and were adequately treated.

Congenital syphilis is also the second leading cause of preventable stillbirth globally, preceded only by malaria.

Testing and treatment

The number of women and infants affected by syphilis remains unacceptably high. It is crucial that all women are provided with early syphilis screening and treatment as part of high-quality antenatal care, to enable a positive pregnancy experience. In addition, health systems and programmes need to ensure that all women diagnosed with syphilis, as well as their infants, are effectively treated – and that their sexual partners are reached for testing and treatment. Countries can also work to reduce syphilis prevalence across populations, by ensuring that testing, treatment and partner referral for the infection are implemented, beyond antenatal care.

Syphilis is inexpensive to detect and treat, making it a possible “easy win” in terms of cost, feasibility and speed of scale-up. Investing in screening and treatment for syphilis in pregnant women ranks as one of the most cost-effective antenatal interventions. 

Syphilis testing and treatment coverage among pregnant women is still low in many countries – lower than antenatal HIV testing and treatment. While WHO recommends testing at the point of care using rapid diagnostic tests, most countries still rely on laboratory testing for syphilis. WHO recommends a simple, proven and inexpensive  dual test for syphilis and HIV to improve the quality, acceptability and uptake of testing and treatment of maternal syphilis. The dual rapid diagnostic tests offer the opportunity to immediately close the gap between HIV and syphilis testing among pregnant women, and to accelerate elimination of mother-to-child transmission of both syphilis and HIV.

WHO is working towards eliminating mother-to-child transmission of HIV, syphilis and hepatitis B. Success has already been made in several parts of the world, with 15 countries and territories validated by WHO as having eliminated mother-to-child transmission of syphilis and/or HIV in recent years.

Surveillance, monitoring and evaluation

Monitoring the scale-up of screening and treatment for pregnant women remains paramount to measuring progress towards the elimination goal. Measuring how many adults and infants are affected by syphilis with regional- and national-level estimation is crucial to guide the capacity of health systems to strengthen the prevention, detection and treatment of syphilis.

WHO has developed a Congenital Syphilis Estimation Tool, primarily for use by country programme managers to strengthen monitoring and evaluation within existing national systems and for use in validation. In addition, this tool is intended for regional and global professionals to support strengthened monitoring and evaluation (M&E) of elimination of congenital syphilis efforts. This tool is a focused synthesis of existing WHO guidance on surveillance, monitoring, and evaluation.