Most people first hear about a low-lying placenta at their 20-week anomaly scan — and most will go on to have a completely normal third trimester once the placenta has moved. This guide covers what the finding means, when it resolves, what confirmed placenta praevia involves, the accreta spectrum, symptoms to act on, and what birth looks like when praevia is confirmed.
🌿 Open WiseMama — freeThe placenta — the organ that provides oxygen and nutrients to the baby throughout pregnancy — normally attaches to the upper part of the uterine wall, away from the cervix. Placenta praevia occurs when the placenta attaches in the lower segment of the uterus, either partially or completely covering the internal cervical os — the opening of the cervix through which the baby must pass during a vaginal birth.
Current RCOG terminology uses two main categories:
Confirmed placenta praevia at term affects approximately 0.5% of pregnancies — roughly 1 in 200. However, up to 6% of pregnancies appear to have a low-lying placenta at the 20-week anomaly scan. The vast majority of these resolve as the pregnancy progresses — which is why a finding at 20 weeks is not the same as a diagnosis of placenta praevia.
Vasa praevia is a different and rarer condition where fetal blood vessels run unsupported through the membranes over the cervical os — rather than the placenta itself covering it. It is extremely dangerous in labour, because the blood vessels can rupture when the membranes break, causing rapid fetal blood loss. It is diagnosed on ultrasound and managed with planned caesarean section before labour begins. It is mentioned here because it is sometimes confused with placenta praevia but requires specific management of its own.
If you have just been told at your 20-week anomaly scan that your placenta is low-lying, the most important thing to understand is that this is a common finding at this gestation — and in the large majority of cases, it resolves by the third trimester without any intervention or special management.
The placenta does not actually move — but it appears to, because of how the uterus grows. As the uterus expands significantly in the second and third trimesters, the upper segment grows much more than the lower segment. The part of the uterine wall where the placenta is attached moves upward and away from the cervix relative to the os. A placenta that appears to cover or be adjacent to the os at 20 weeks may be entirely clear of it by 32–36 weeks.
Studies show that up to 90% of placentas identified as low-lying at the 20-week scan are in a normal position by 32–36 weeks. The earlier in gestation and the higher the placenta above the os, the more likely it is to resolve completely.
RCOG guidance recommends a follow-up transvaginal ultrasound scan at 32 weeks to reassess placental position. A transvaginal (internal) scan gives a more accurate measurement of the distance between the placental edge and the internal os than an abdominal scan, particularly in the third trimester. If the placenta is still low-lying at 32 weeks — within 20mm of the os — a further scan at 36 weeks is recommended before final decisions about birth planning are made.
One specific combination warrants more urgent assessment: an anterior (front-wall) low-lying placenta in a woman who has had a previous caesarean section. The previous scar on the lower uterine segment creates an area where placental attachment can be abnormally deep — increasing the risk of placenta accreta (see below). If you have had a previous caesarean and are told your placenta is low-lying on the anterior wall, you will be referred to a specialist team for additional assessment, including possible MRI.
A low-lying placenta at 20 weeks is common and usually resolves. Avoid penetrative sex and internal examinations until your follow-up scan confirms position. Report any vaginal bleeding to your maternity unit immediately. Your follow-up scan at 32 weeks will determine whether further management is needed.
While placenta praevia can occur in any pregnancy, several factors are associated with higher risk. Understanding them is useful both for awareness and for explaining why some pregnancies are monitored more closely than others.
The most significant modifiable risk factor. Each previous caesarean section leaves scar tissue on the lower uterine segment, which can attract placental attachment in subsequent pregnancies. The risk of placenta praevia increases with the number of previous caesareans — and the risk of the more serious placenta accreta spectrum disorder increases substantially with each additional caesarean when praevia is also present.
Any previous surgery involving the uterus — including myomectomy (fibroid removal), previous uterine curettage (D&C), or endometrial ablation — can increase the risk through similar mechanisms of scarring affecting implantation.
Multiple pregnancy (twins or more); advanced maternal age; high parity (having had many pregnancies); previous placenta praevia; smoking during pregnancy; and pregnancy conceived via IVF are all associated with modestly elevated risk.
Placenta accreta spectrum (PAS) is the most serious complication associated with placenta praevia, and it deserves specific attention — particularly for women who have had previous caesarean sections.
Normally, the placenta separates cleanly from the uterine wall after the baby is born — this is the third stage of labour. In placenta accreta spectrum, the placenta attaches abnormally deeply and cannot separate normally, causing catastrophic haemorrhage at delivery.
There are three degrees of severity:
The combination of anterior placenta praevia and previous caesarean section is the highest-risk scenario. The risk increases dramatically with the number of previous caesareans:
Ultrasound is the primary diagnostic tool — the placenta may show irregular lacunae (spaces), loss of the normal clear zone between placenta and uterine wall, and increased vascularity. MRI can provide additional information about depth of invasion, particularly for percreta. If PAS is suspected, referral to a specialist multidisciplinary team — at a centre with high-volume experience — is essential.
Management typically involves planned caesarean delivery at 34–36 weeks (before spontaneous labour begins), performed by a specialist team with interventional radiology, urology, and blood bank support available. In many cases of accreta, hysterectomy at the time of caesarean is the safest management — the placenta is left in place and the uterus removed with placenta attached, avoiding the haemorrhage of attempted manual removal. This is a significant decision with profound implications for future fertility, and it requires careful multidisciplinary discussion in advance.
The classic presentation of placenta praevia is painless, bright red vaginal bleeding in the second or third trimester — typically after 28 weeks, though it can occur earlier. The absence of pain distinguishes it from placental abruption (where the placenta separates from the uterine wall, causing painful bleeding).
Bleeding from placenta praevia is typically bright red and may range from a small amount of spotting to a heavy gush. It often stops spontaneously within hours. This spontaneous resolution does not mean the praevia has resolved — the placental position is unchanged, and further bleeding episodes are likely. The bleeding can be triggered by sexual intercourse, an internal examination, or physical activity, though it frequently occurs without any precipitating cause.
Some women with confirmed placenta praevia never experience bleeding — the condition is diagnosed on ultrasound without any symptoms.
Any vaginal bleeding in the second or third trimester requires immediate contact with your maternity unit — the same day, or by calling 999 if bleeding is heavy or you feel faint or unwell. Do not wait to see if it settles. You will be assessed, your blood pressure and fetal wellbeing will be checked, an IV line will be placed as a precaution, your blood group will be confirmed, and anti-D will be given if you are Rhesus negative.
Do not drive yourself to hospital if you are bleeding — ask someone else to drive you or call an ambulance.
Even if the bleeding stops. Even if it was only a small amount. Even if you feel otherwise well. Painless bright red bleeding in the second or third trimester always needs same-day assessment. Call 999 if bleeding is heavy, if you feel faint, or if you have abdominal pain alongside the bleeding.
Once placenta praevia is confirmed, management focuses on prolonging the pregnancy as safely as possible while monitoring both mother and baby closely and preparing for delivery.
Penetrative sex and internal vaginal examinations are contraindicated in confirmed placenta praevia — both can trigger bleeding. This restriction begins from confirmation of praevia and continues until after delivery.
Regular appointments with consultant-led care, haemoglobin monitoring (to check for and treat anaemia from blood loss), growth scans to monitor fetal wellbeing, and blood group and antibody screening (in case transfusion is needed). Women with confirmed major praevia who have experienced bleeding may be admitted to hospital for close monitoring, particularly from around 34 weeks.
Repeated small bleeds can cause iron deficiency anaemia, which is both unpleasant and reduces the mother's resilience for the larger blood loss expected at delivery. Oral iron supplements are usually prescribed; IV iron may be given if oral supplements are not sufficient.
If there is a significant bleeding episode before 34 weeks, a course of corticosteroids (betamethasone) is given to promote fetal lung maturity in case early delivery becomes necessary.
Women with major placenta praevia are often admitted to hospital from around 34–36 weeks, or earlier following a significant bleeding episode. Proximity to the operating theatre matters — in the event of a large bleed, time to delivery is critical. Some hospitals manage women with stable praevia at home until 36–37 weeks with clear instructions about when to call an ambulance; others prefer inpatient management from 34 weeks. Your consultant will discuss the approach appropriate for your specific situation.
Major placenta praevia requires delivery by caesarean section — there is no safe alternative. The placenta overlying the cervix means that as the cervix dilates in labour, the placenta tears and massive haemorrhage occurs. This is a life-threatening emergency that is entirely prevented by planned caesarean delivery before labour begins.
RCOG guidance recommends planned caesarean section at 36–37 weeks for uncomplicated major placenta praevia — balancing the risk of spontaneous bleeding before this gestation against the risks of prematurity. If there has been significant bleeding or the situation is unstable, delivery may be planned earlier. For suspected placenta accreta spectrum, delivery at 34–36 weeks is typically planned, at a specialist centre with a multidisciplinary team in place.
A praevia caesarean is a more complex procedure than a standard lower-segment caesarean section — the placenta may be in the way of the standard incision, the blood supply in the lower uterine segment is greater, and blood loss is typically higher. A consultant obstetrician performs the procedure, with anaesthetic consultant involvement, specialist nursing staff, and ready access to blood products. Most women have a regional anaesthetic (spinal or epidural); general anaesthetic may be used in emergencies.
Blood loss at a praevia caesarean is significantly higher on average than a standard caesarean. Transfusion may be required. Cell salvage — a technique that collects and reinfuses your own blood during surgery — is commonly used. Discussing what to expect in terms of blood loss and transfusion with your obstetric team in advance reduces the shock of these measures when they are used.
Recovery from a praevia caesarean is the same as from any caesarean section, though it may be slightly more complex if there was significant blood loss or accreta management. The WiseMama caesarean birth guide covers caesarean recovery in full. If you were admitted to hospital before delivery, you will have already been away from home for some time — practical planning for the return home, particularly with regard to older children and support, is important.
The months between a low-lying placenta diagnosis at 20 weeks and confirmation of position at 32 weeks can be a prolonged period of uncertainty — neither fully diagnosed nor fully reassured, with restrictions on activity and an awareness that bleeding could occur. For those with confirmed praevia, the third trimester involves multiple hospital visits, possible admission, and an entirely different birth plan to the one imagined. This is genuinely hard to navigate alongside the normal demands of late pregnancy, work, and family life.
Placenta praevia fundamentally changes the birth experience — for many women this means the loss of a hoped-for vaginal birth, a home birth, or a birth centre birth. Grief for the birth experience that will not happen, alongside anxiety about the one that will, is a normal and valid response. It is worth raising with your midwife or consultant if it is significantly affecting your wellbeing.
Having had placenta praevia increases the risk of praevia in subsequent pregnancies. The risk is not overwhelming — most women who have had praevia in one pregnancy do not develop it again — but it is worth flagging to your booking midwife early in any subsequent pregnancy so that placental position is assessed promptly at the 20-week scan and, if indicated, earlier.
If you had a caesarean section for placenta praevia, the uterine scar adds a risk factor for both praevia and accreta in subsequent pregnancies. The more caesareans you have, the higher this cumulative risk. This is worth discussing with your consultant when considering future pregnancies — not as a reason to avoid them, but to plan appropriately for the additional monitoring they may need.
The RCOG and NHS websites have patient information leaflets on placenta praevia and accreta. For peer support from others who have been through the same experience, Mumsnet's pregnancy complications board has active threads, and the International Placenta Accreta Foundation (placenta-accreta-foundation.org) provides resources specifically for accreta spectrum. Your midwife and consultant team are the appropriate clinical sources for questions specific to your pregnancy.
I was told at 20 weeks and spent two months convinced I had a serious problem. At 32 weeks the placenta was 35mm from the os — completely normal. The follow-up scan was the most relieving appointment of my pregnancy. Most people get that scan result. I just wish I'd known that before the two months of anxiety.