Background This cross-sectional study was conducted to characterise P. falciparum infections matched in peripheral, placental and cord blood among Congolese women at delivery receiving 1, 2 or more doses of sulfadoxine-pyrimethamine. The cross-sectional study was conducted in a Southern district of Brazzaville, Republic of the Congo, between March 2014 and April 2015.

Methods Peripheral and placental blood samples were collected for P. falciparum infection investigation by microscopy and nested polymerase chain reaction (PCR), using P. falciparum merozoite surface protein-2 (msp2) gene as marker.

Results Of the 370 pregnant women recruited, only 7.3% peripheral and 2.7% placental blood samples were found smear-positive for P. falciparum by microscopy. All isolates from cord blood were microscopy-negative. However, the prevalences of submicroscopic P. falciparum infections (detectable only by PCR) were 25.4%, 16.7% and 9.4% in peripheral, placental and cord blood respectively. The frequency of 3D7 msp2 alleles was the highest (>60%) whatever the blood considered. We found a high prevalence of submicroscopic infection in pregnant women associated with a high genetic diversity of P. falciparum isolates. The multiplicity of infection ranged between 1.2 and 1.4 irrespective of the blood compartment, and it showed no significant association with maternal age (p=0.3), gravidity (p=0.1) or sulfadoxine-pyrimethamine (p=0.3).

Conclusion In summary, this study showed that there is a high prevalence of submicroscopic infection and a high genetic diversity of Plasmodium falciparum strains in Congo. This diversity varies according to maternal, placental and umbilical cord blood. Age, gravidity and doses of preventive treatment based on sulfadoxine-pyrimethamine do not interfere with the multiplicity of infections.