Newly infected mothers are more likely to pass HIV on to babies

Not out of the woods yet. US Army Africa, CC BY

Women are twice as likely than men to contract HIV during intercourse and according to the authors of a new US study, women who contract HIV while pregnant may be especially vulnerable and more likely to pass on the virus to their child than women who already had the virus.

Published in PLOS Medicine, the researchers analysed the results of 19 studies (all carried out in sub-Saharan Africa) that had estimated the rate of new, or “incident”, HIV infections in pregnant women or those who had given birth in the preceding 12 months. They then calculated a “pooled” rate of new HIV infections during pregnancy.

The pooled rate suggested women were at high risk of contracting HIV while pregnant. This rate was similar to other high risk groups, such as female sex workers and discordant couples (where one partner is HIV-positive and the other HIV-negative).

Newly infected

All people who are newly infected with HIV have extremely high levels of virus in their bodies during the first few months after becoming infected, but are usually unaware they are infected. This puts them at higher risk of transmitting the virus – either to their sexual partners or, in the case of pregnant or breastfeeding women, to their babies. This is clearly important in countries like Swaziland, the country most affected by HIV/AIDS and where a quarter of the adult population lives with HIV. It also has the highest rates of new HIV infections in pregnancy.

In general, women are both biologically and behaviourally more vulnerable to HIV infection and around twice as likely as male partners to become infected during sex. But it is still uncertain whether pregnant women are more vulnerable to HIV than they would be if they were not pregnant. The study was a systematic review, meaning it looked at existing studies and was limited by the number of studies that included relevant information on pregnant and non-pregnant women.

In two of five studies that could address the specific question of risk (one carried out across seven African countries and the other in South Africa), pregnant women were around twice as likely to become HIV infected than non-pregnant women. But when the researchers combined the findings of all five, they didn’t find a higher risk of becoming infected with HIV in pregnant women compared with non-pregnant women. So while pregnant women can be deemed high risk, the evidence of whether they are at more risk than non-pregnant women is still inconclusive.

One potential explanation for an increased vulnerability to HIV in pregnancy may be biological, including changes in hormonal levels and in the immune system. However, behavioural factors are also likely to be important. The increased risk of HIV infection during pregnancy identified in some studies may be partly explained by more women having unprotected sex because they are no longer using condoms while pregnant. Other factors may include behavioural changes in male partners, for example, who may initiate or increase sexual activity outside the relationship while their partner is pregnant or breastfeeding.


One important finding from the study is that babies were more likely to be infected if their mothers became HIV-infected during pregnancy or while breastfeeding compared with babies whose mothers had longer-established or “chronic” HIV infection.

Transmission between mother and child can take place in the womb, during delivery and through breastfeeding. But only once an HIV-positive pregnant woman is identified can she be offered effective interventions like anti-HIV drugs to reduce the risk of transmission.

Antenatal HIV testing is the cornerstone of WHO and UNAIDS strategies to prevent HIV-positive mothers from passing on the virus to their babies and to eliminate new HIV infections in children. In the UK and Ireland, mother to child HIV transmission is at an all time low, according to a study in journal AIDS, thanks to a combination of factors including the very high uptake of antenatal HIV screening and early antenatal combination antiretroviral therapy. This shows how important intervention is.

The findings from the PLOS Medicine study draw attention to the fact that women are at high risk of becoming HIV infected during pregnancy and the period after birth in countries where HIV is common. The authors recommend that in these settings, repeat HIV testing should be offered to women in late pregnancy and to women who have delivered babies but haven’t received antenatal care. Although international guidelines recommend that repeat testing should be happening, in practice this has usually not been the case.

Repeat HIV testing could identify more women with new HIV infections who could then receive anti-HIV drugs to reduce the chance of them passing the infection on to their child. It would also give more opportunity for HIV care and treatment, and testing for their partners.

The most effective approach to preventing new infant HIV infections is to prevent women of childbearing age from becoming HIV infected in the first place. The authors also suggest that efforts to prevent HIV infection among pregnant and new mothers should be strengthened, for example, with additional counselling on the benefits of condom use while pregnant or breastfeeding.