They added doctors should discuss with women the safest way to give birth.
The conditions can lead to heavy bleeding and sometimes put mother and baby’s lives at risk.
TV personality Kim Kardashian developed placenta accreta when pregnant with her daughter North West.
TV presenter Amanda Holden suffered from the rare placenta praevia-accreta when expecting her youngest daughter Hollie.
Placenta praevia happens when the placenta attaches in the lower part of the uterus (womb), sometimes completely covering the cervix (neck of the womb).
Placenta accreta is a rare but serious condition when the placenta is stuck to the muscle of the womb and/or to nearby structures such as the bladder.
Both conditions can cause heavy bleeding, particularly towards the end of the pregnancy, because the placenta is low down in the uterus.
This can sometimes require blood transfusions and lead to the loss of the uterus.
The RCOG has produced an online leaflet with information for women who may develop or have these conditions.
It says the rates of placenta praevia and placenta accreta have increased and are likely to continue to increase as a result of rising rates of caesarean births and the use of assisted reproductive technology, such as IVF.
This is because potential scars in the uterus created by caesareans and fertility treatment mean any subsequent pregnancies are more likely to implant in the scar, causing problems with the placenta.
The highest rates of complication for both mother and baby occur when these placenta complications are only diagnosed at delivery, the RCOG said.
And it encouraged more scans and specialised care to help reduce any risks.
‘Discuss risk factors’
Prof Eric Jauniaux, lead author of the guidelines, said: “We hope this updated guidance will support healthcare professionals during discussions with women and their partners who may be considering assisted reproduction, particularly IVF, or an elective caesarean birth.
“These are both risk factors that can increase risk of developing these serious complications of the placenta.”
Prof Jauniaux, professor of obstetric and foetal medicine at University College London, said the guidelines should also help doctors to diagnose the complications early and discuss the safest way for women to give birth.
Dr Geeta Kumar, chair of the RCOG patient information committee, said: “It is important for women to be aware of these complications of the placenta, but they should be reassured that these are very rare and specialist care is available.
“Early diagnosis is key to reduce harms associated with placenta praevia and accreta to ensure the safest possible outcome for mother and baby.”
What does the placenta do?
The placenta develops together with the baby in the uterus during pregnancy.
It attaches to the wall of the uterus and provides a connection between mother and baby.
Oxygen and nutrients pass from the woman’s blood through the placenta and into the baby’s blood.
The placenta is delivered shortly after a baby is born, and is sometimes called the afterbirth.
How common is placenta praevia?
It is more common if a woman has had previous caesareans, fertility treatment or if they smoke.
If you have placenta praevia, your baby will probably need to be born by caesarean.
One in 200 women have the condition at the end of their pregnancy.
How common is placenta accreta?
It is a complication which affects between one in 300 and one in 2,000 pregnancies.
This is more common if a woman has previously had a caesarean.
It may cause heavy bleeding at the time of birth.
What is vasa praevia?
Vasa praevia is a very rare condition affecting between one in 1,200 and one in 5,000 pregnancies.
It is where blood vessels travelling from your baby to your placenta, unprotected by placental tissue or the umbilical cord, pass near to the cervix.
If these blood vessels tear, this can be very dangerous for your baby.