Placenta

• Document placental location, for example, anterior, posterior, fundal.
• Ask about any previous caesarean section and document placental location in relation to the scar.
• Image in transverse and longitudinal planes.
• Measure distance of the lower placental margin/marginal sinus from the internal os. Consider a TV scan if the lower margin cannot be well visualised in relation to the internal os.
• A full bladder can simulate low-lying placenta; if in doubt, get the woman to empty her bladder.
• Less than 20 mm is considered low lying. Third-trimester follow-up is recommended.
• Assess placental cord insertion and location.
• In case of velamentous cord insertion, placenta previa or succenturiate lobe, careful assessment of the internal os is required with colour Doppler, with a low threshold for TV imaging, to exclude vasa previa.

• Placenta previa / low-lying placenta (see below)
• Velamentous cord insertion
• Marginal cord insertion
• Succenturiate lobe
• Vasa previa (see below)
• Suspected placenta accreta (see below)
• Placental mass, for example, chorioangioma.

The placenta is inserted wholly or partially into the lower segment of the uterus. Terms used to describe types of placenta previa include:

• complete placenta previa – the placenta completely covers the internal cervical os
• partial placenta previa – the placenta is partly over the cervix
• marginal previa – the placenta is near the edge of the cervix
• low-lying placenta – the placenta/marginal sinus is 20 mm or less from the internal cervical os.

Vasa previa occurs when exposed fetal vessels within the amniotic membranes cover, or are within 20 mm of, the internal cervical os.

This is a clinically significant condition as the vessels are from the fetoplacental circulation, and rupture will lead to rapid fetal exsanguination and death.

There are two types of vasa previa.

• Type I occurs with velamentous insertion of the umbilical cord into the placenta.
• Type II occurs with velamentous fetal vessels connecting the placenta to a succenturiate placental lobe.

If a low-lying placenta and/or placenta previa (see above) is found at the anatomy scan, recommend follow-up at approximately 32 weeks gestation, which may include a TV scan.

If there is evidence of vasa previa (see above) at the anatomy scan, report the finding and recommend specialist review and follow-up scans.

Placenta accreta / abnormally invasive placenta (AIP) is a disorder of placental implantation where there is invasion of placental tissue into the uterine wall. It is associated with a high risk of maternal and fetal morbidity and mortality.

Careful assessment is recommended with women who have had a previous caesarean section where the placenta overlies the scar, and if there are any concerns, tertiary opinion is recommended.

Traditionally, the extent may be mild, moderate or severe. However, this can be difficult to assess accurately on ultrasound.

• Accreta – placental extension through the thinned decidua basalis without myometrial invasion
• Increta – invasion into the myometrium
• Percreta – invasion through the myometrium with breach of the serosa, with or without invasion into adjacent structures, for example, into the bladder.

The most common risk factors are a previous caesarean section and placenta previa (see above).

Features of abnormal placental implantation include:

• irregular placental lakes/lacunae
• focal bulging of the placenta
• abnormal myometrial-bladder wall interface
• loss of retroplacental hypoechoic space
• disordered vascularity.

For first-trimester features, see Appendix 2: Low gestational sac in the first trimester with previous caesarean section.

• Parity
• Number of previous caesarean sections
• Number of classical caesarean sections
• Number of previous surgical evacuations (including termination of pregnancy, TOP)
• Previous uterine surgery (eg, myomectomy, endometrial ablation)
• Past history of accreta/AIP.

Greyscale features

• Loss of clear zone – loss or irregularity of the hypoechoic retroplacental ‘clear’ zone in the myometrium underneath the placental bed.
• Myometrial thinning overlying the placenta to <1 mm, or undetectable.
• Abnormal placental lacunae – often numerous, including some that are large and irregular, often containing turbulent flow visible on greyscale imaging.
• Bladder wall interruption – loss or interruption of the echogenic bladder wall between the uterine serosa and the bladder lumen.
• Placental bulge – deviation of the uterine serosa away from the expected plane, caused by abnormal bulge of placental tissue into a neighbouring structure, typically the bladder. The uterine serosa appears intact, but the outline is distorted.
• Focal exophytic mass – placental tissue is evident breaking through the uterine serosa and extending beyond it. This is most often seen within the filled bladder.

Colour Doppler ultrasound features

• Uterovesical hypervascularity – striking colour Doppler flow between the myometrium and posterior wall of the bladder.
• Subplacental hypervascularity – striking colour Doppler flow within the placental bed.
• Bridging vessels – vessels extending from the placenta, across the myometrium ± beyond the serosa into the bladder or other organs; these often course perpendicular to the myometrium.
• Placental lacunae feeder vessels – vessels with high-velocity blood flow leading from the myometrium into the placental lacunae, with turbulence.
• Parametrial involvement – suspicion of invasion into the parametrium.

• The lower segment of the uterus should be carefully assessed for possible placenta accreta at the anatomy scan in women who have had previous caesarean sections, placenta previa or both, particularly if the placenta is anterior.
• Pay particular attention to the anterior uterine wall-bladder wall interface.
• Assess with a combination of TV and TA scan with a high-frequency transducer, such as a linear array.
• Consider TV scan with the bladder partially full to provide an interface with the myometrium.
• Longitudinal and transverse images (without and with colour Doppler).
• Assess for above features of AIP.

For more detail, see:

1. FIGO consensus guidelines on placenta accreta spectrum disorders: Prenatal diagnosis and screening (Jauniaux et al 2018)
2. Proposal for standardized ultrasound descriptors of abnormally invasive placenta (AIP) (Collins et al 2016)
3. Pro forma for ultrasound reporting in suspected abnormally invasive placenta (AIP): an international consensus (Alfirevic et al 2016).

• Report placental location (anterior, posterior) and distance from os.
• Report suspicious ultrasound findings (as at Ultrasound features above).

If there are features suspicious for placenta accreta at the anatomy scan, recommend specialist referral and follow-up detailed scan.