Three windows, very unequal
Without any intervention, the risk that a woman living with HIV passes the virus to her child is 15 to 45%. That total splits across three distinct moments, and it is the second that carries most of the weight:
| During pregnancy | Roughly a fifth to a quarter of transmissions. The placenta is an effective barrier as long as it is not damaged. |
|---|---|
| During labour and delivery | The main window: on the order of half of all transmissions. The baby is directly exposed to maternal blood and genital secretions, and contractions increase micro-exchanges between the two circulations. |
| During breastfeeding | A quarter to nearly half of transmissions where breastfeeding is prolonged. It is the only window that lasts for months. |
With full management, that risk falls to about 1% without breastfeeding and 2% with breastfeeding on treatment — and below 0.5% in countries with systematic antenatal care. Put plainly: a pregnant woman living with HIV who is diagnosed and treated has better than a 99% chance of delivering a baby free of infection.
The one number that matters
The whole approach rests on a single variable: the mother's viral load. It determines the mode of delivery, whether an intravenous drug is needed during labour, what the newborn receives, and whether breastfeeding is an option. The goal is an undetectable viral load in the third trimester, sustained through delivery.
Antiretroviral treatment is continued if already in place, or started as soon as the diagnosis is made — including late in pregnancy, where it remains highly effective. It is never interrupted during pregnancy.
During labour and delivery
This is where most of the risk sits, and where decisions are taken within hours.
Vaginal delivery or caesarean
When the viral load is controlled near term, vaginal delivery is recommended: a caesarean adds no benefit and carries its own risks. That is a complete reversal from the 1990s, when routine caesarean was the rule.
The viral load threshold that triggers a planned caesarean is one of the points where national guidelines diverge most: some set it at 50 copies per millilitre, others at 400, others at 1,000. Ask explicitly which threshold your maternity unit applies.
Where a caesarean is indicated, it is scheduled at around 38 weeks — that is, before labour starts and before the membranes rupture. A caesarean decided as an emergency after hours of labour, or after the waters have broken, loses most of its benefit: the exposure has already happened.
Treatment during labour
If the viral load is detectable, unknown, or treatment was started late, an intravenous zidovudine infusion is given from the onset of labour until the cord is clamped. It is not needed where the viral load has remained undetectable.
What is avoided
While the viral load is not controlled, anything that breaks the baby's skin or prolongs exposure is avoided:
- Fetal scalp electrodes and fetal blood sampling from the scalp.
- Instrumental delivery, particularly by ventouse.
- Artificial rupture of the membranes, and more generally a prolonged interval between rupture and delivery: beyond about four hours the risk rises measurably.
- Episiotomy where it is not strictly necessary.
Preterm labour and premature rupture of the membranes both raise the risk and shift management into an accelerated mode.
If HIV status is unknown on arrival
A woman arriving in labour without a recent test is offered a rapid test on the spot. If it is positive, treatment for the mother and prophylaxis for the baby are started immediately, without waiting for the confirmatory test. Management begun at the last moment still reduces the risk substantially: it is never too late to act.
The hours and weeks that follow
- The newborn is washed promptly, and blood tests or injections are deferred until after that first bath.
- Antiretroviral prophylaxis is given to the baby, longer and stronger the higher the risk was: zidovudine alone for two to four weeks where risk is low, a combination where it is not. Several recent guidelines omit it altogether where the mother's viral load stayed undetectable throughout pregnancy.
- The baby is tested by virological test (PCR), never by antibody test: maternal antibodies cross the placenta and remain detectable in the child for up to eighteen months. A baby who tests "HIV-positive" on an antibody test at three months is not necessarily infected.
- The schedule involves several PCR tests — typically at birth, at one to two months, then at four to six months — with a final confirmation. Two negative results after all exposure has ended rule out infection.
- BCG vaccination is deferred, where it is routine, until infection has been excluded.
Infant feeding: two policies, one logic
This is the third window, and the longest. The answer differs between countries not because the science differs, but because the competing risk does:
- Where clean water and formula are safe, available and affordable, formula feeding removes the risk of transmission after birth entirely. For a long time this was the only option offered.
- Where they are not, the World Health Organization recommends exclusive breastfeeding for six months on antiretroviral treatment, then continued alongside complementary foods. The reasoning is arithmetic: deaths from diarrhoea and malnutrition linked to formula outweigh the residual transmission risk.
- A recent change: several high-income countries now support the choice to breastfeed where the viral load is sustainably undetectable, through shared decision-making and close follow-up. The risk is then under 1%, but not zero.
Never mix breast and bottle
Mixed feeding is the highest-risk configuration: formula irritates the infant's gut lining and makes it easier for virus present in breast milk to cross. Whichever option is chosen, it should be exclusive.
Conceiving without risk to a partner
Where the partner living with HIV has had an undetectable viral load for at least six months, the virus is not sexually transmitted: natural conception carries no risk to the other partner. Where that is not the case, assisted reproduction and pre-exposure prophylaxis for the HIV-negative partner are well-established alternatives.
Worth remembering
Antenatal testing is the decisive link: it is what makes everything else possible. A test offered in the first trimester, and repeated later where there has been exposure, turns a one-in-four risk into a risk below one in a hundred.
What differs between countries
The principle is the same everywhere; the thresholds, the protocols and above all the access are not.
- The caesarean threshold — 50, 400 or 1,000 copies per millilitre depending on national guidance. It is the most concrete divergence between neighbouring countries.
- Infant feeding policy — formula by default, breastfeeding recommended on treatment, or shared decision-making based on viral load.
- Antenatal testing — routinely offered, offered on an opt-out basis, or left to the clinician; repeated in the third trimester or not.
- Cost — treatment, monitoring, formula and infant testing are fully covered in some countries, partly in others.
- Actual coverage — this is what explains most remaining transmissions worldwide: not treatment failure, but an absent test or a break in the drug supply.
What applies where you live — protocols, services, costs and contacts — is set out in the HIV country pages linked below.