Placenta Praevia and Low-Lying Placenta

Two diagnoses, one measurement

The RCOG separates them by distance: "This condition is known as low-lying placenta if the placenta is less than 20 mm from the cervix or as placenta praevia if the placenta completely covers the cervix."

That 20 mm figure is the number your care hangs on, and it is measured by transvaginal ultrasound, which the RCOG describes as safe for you and your baby and more accurate than an abdominal scan for this purpose.

Green-top Guideline No. 27a explains why these conditions get so much attention: they "are associated with high maternal and neonatal morbidity and mortality, due to antepartum haemorrhage and/or major obstetric haemorrhage during birth and preterm birth", and rates of both praevia and accreta "have increased" as caesarean and IVF rates have risen.

Most low-lying placentas move

This is the most important sentence on this page if you have just had your 20-week scan. The RCOG: "9 out of 10 women with a low-lying placenta at their 20-week scan will no longer have a low-lying placenta when they have their follow-up scan, and only 1 in 200 women overall will have placenta praevia at the end of their pregnancy."

The HSE gives the same figures: "For 9 in 10 women with a low-lying placenta, the placenta moves up in the womb by the third trimester and does not affect the birth", and "placenta praevia in the third trimester happens in 1 in 200 pregnancies".

The placenta does not migrate. The lower part of the uterus grows and lengthens, carrying the placenta upward relative to the cervix. One group is less likely to see this happen: "If you have previously had a baby by caesarean, the placenta is less likely to move upwards."

Your scan schedule

The follow-up is not open-ended, and knowing the dates lets you check they have been booked:

• 20 weeks: placental position recorded at the routine anomaly scan.
• 32 weeks: "If your placenta is low lying at your 20-week scan, you will be offered a follow-up scan at 32 weeks of pregnancy to see whether it is still low lying. This may include a transvaginal scan."
• 36 weeks: "You should be offered a further ultrasound scan at 36 weeks if your placenta is still low lying."

Your cervical length may also be measured at the 32-week scan, "to predict whether you may go into labour early and whether you are at increased risk of bleeding".

What bleeding means when you have this diagnosis

Bleeding from placenta praevia "is usually painless and may occur after having sex", and it "can be very heavy, sometimes putting both you and your baby's life at risk".

The RCOG's instruction is unambiguous: "If you know you have a low-lying placenta, you should contact the hospital straight away if you have any vaginal bleeding, contractions or pain." The HSE words the same advice as urgent advice to contact your maternity unit or hospital immediately for any vaginal bleeding, tummy pain or contractions.

The critical point is the one the RCOG makes about people who have had no symptoms at all: "Even if you have had no symptoms before, there is a small risk that you could bleed suddenly and heavily, which may mean that you need an emergency caesarean." A quiet pregnancy so far is not a prediction.

Green-top Guideline No. 63 defines antepartum haemorrhage as "bleeding from or in to the genital tract, occurring from 24+0 weeks of pregnancy and prior to the birth of the baby", names placenta praevia as one of its two most important causes, and notes that APH "complicates 3-5% of pregnancies".

Steroids, preterm birth and hospital admission

Placenta praevia raises the chance of a preterm birth, so "you may be offered a course of steroid injections between 34 and 36 weeks of pregnancy to help your baby to become more mature". If you go into labour early, tocolysis may be offered to try to buy time for that course to be completed.

Admission is individualised rather than automatic. The RCOG says additional care, "including whether or not you need to be admitted to hospital, will be based on your individual circumstances." The HSE describes a common Irish pattern: "At week 34 of your pregnancy, you may need to go into hospital until you give birth", earlier if you have had repeated bleeding.

Iron, blood and consent

Anaemia is worth correcting in advance rather than afterwards. The RCOG: "You should try to avoid becoming anaemic during pregnancy by having a healthy diet and by taking iron supplements if recommended by your healthcare team. Your blood haemoglobin levels will be checked at regular intervals during your pregnancy." The HSE also lists an iron supplement among the extra care you may need.

You may need a blood transfusion if bleeding is heavy. If you would decline blood or blood products for religious or cultural reasons, the HSE advises telling your obstetrician or midwife early so that it is recorded in your notes and planned around.

How the birth is planned

The decision rule is the same 20 mm: "If the edge of your placenta is less than 20 mm from the entrance to the cervix on your scan at 36 weeks, a caesarean will be the safest way for you to give birth. If the placenta is further than 20 mm from your cervix you can choose to have a vaginal birth."

On timing: "Unless you have heavy or recurrent bleeding, your caesarean will usually take place between 36 and 37 weeks. If you have had vaginal bleeding during your pregnancy, your caesarean may need to take place earlier than this." A senior obstetrician and anaesthetist should be present.

Placenta accreta, briefly

Accreta is a separate condition that can coexist: "a rare but serious condition when the placenta is stuck to the muscle of your womb and/or to nearby structures such as your bladder. This is more common if you have previously had a caesarean." Green-top Guideline No. 27a notes that "the highest rates of complication for both the woman and their newborn are observed when these conditions co-exist and are only diagnosed during birth" - which is the whole argument for the scan schedule above.