Women who have their first child by caesarean are more likely to have placenta-related problems in their second pregnancy, research suggests.
Problems with the placenta can cause bleeding during pregnancy
The risk of a placenta attaching low down in the womb or rupturing increases by almost 50%, data from 5 million pregnancies shows.
Both conditions cause bleeding during pregnancy, the report warned.
Experts said women should be informed of the risk if they are considering having an elective caesarean.
BJOG, an international journal of obstetrics and gynaecology, said researchers analysed data from the US National Centre for Health Statistics between 1995 and 2000.
In mothers who had their first baby by caesarean, the risk of placental praevia - a condition where the placenta attaches over or near the internal opening of the cervix - was 47% higher in their second pregnancy than women who gave birth naturally.
The risk of a second condition, placenta abruption - where the placenta separates from the womb prematurely - was 40% higher in women who had a previous caesarean.
The researchers said scarring inside the womb caused by caesareans may affect the attachment of the placenta in future pregnancies.
In the UK, the rate of placenta praevia is around 0.8% of all births.
Although the numbers are small, both conditions can be very dangerous to both mother and baby.
Study leader Dr Qiuying Yang, from the University of Ottawa, said the study was the largest to date and showed an important link between caesareans and subsequent pregnancy complications.
"More than 1% of pregnancies with a prior caesarean section had one of these events, which had a 50% increase compared to women without previous caesarean section.
"This has important implications on the management of these pregnancies. It also introduces new and important evidence in the debate on the risks of caesarean sections 'on demand'."
Professor Andrew Shennan, consultant obstetrician at St Thomas' Hospital in London and spokesperson for baby charity Tommy's, said problems with bleeding in pregnancy were a daily occurrence for obstetricians.
"We knew already there was an association but the impressive thing about this study is its size, and they've done some really good analysis.
"If someone has a low placenta on a scar, that's a dangerous situation and one of the most frightening in obstetrics.
"It's something that should come into a consultation about caesarean when there is an element of choice involved - we're talking about small numbers, it's something women should be aware of."
Professor Philip Steer, BJOG editor-in-chief, added: "The caesarean section rate in the UK is one of the highest in the world.
"Women need to be informed of the possible risks that can happen throughout the pregnancy and it is important to have their scheduled ultrasound scans to detect if the baby is developing safely.
"Should a woman encounter unusual bleeding during her pregnancy, it is best if she sees the doctor immediately."