The placenta is a discoid , haemochorial organ. It grows linearly throughout the pregnancy to reach an approximate thickness of 2.5cm, and diameter of 22cm by term.While the placenta is often represented as two separate plates divided by a clear interface, in reality the boundary is less distinct.
Image Courtesy Dr Alice Flanagan
The placenta is composed of a chorionic plate (facing the fetus), a basal plate (facing the uterine wall), and an intervening intervillous space. Maternal blood enters this space through spiral arteries to bathe the chorionic villi.
This 3D power doppler image clearly identifies the complex maternal and fetal vasculature throughout the placenta represented by spiral arteries, villous tree, arcuate and uterine arteries.
The placenta should be carefully assessed for its:
Architecture (including myometrial thickness)
Ultrasound review should include placental variations, pathologic conditions, including abnormal placental location, morbid placental adherence , premature placental separation and bleeding, placental cystic spaces, placental masses, and gestational trophoblastic disease.
The amnion surrounding the embryo fuses with the chorion at approximately 14-16 weeks.
At this point the placenta is well formed and visualised easily on transabdominal ultrasound. The placenta should appear homogeneous and uniformly echogenic with a hypoechoic subplacental venous complex located between the placenta and the myometrium.
The placenta may have multiple lobes so be cautious not to assume the largest placenta mass with the cord insertion is the only placenta.
The insertion of the cord into the placenta needs to be assessed for its placement centrally. A marginal or velamentous insertion needs to be documented.
This image demonstrates a posterior placenta at 13 weeks gestation with a normal myometrial contraction.
This image displays the cord inserting centrally into an anterior placenta. The placental myometrial interface is clearly observed.
This image shows a placenta in second trimester located fundally displaying the hypoechoic myometrium behind the placenta. Vascularity should be assessed within the myometrium and the presence of any invasion of the myometrium by the placenta particularly in the case of previous caesarian section.
However, caution is needed when assessing the placenta. Beware of extra lobes with a succenturiate lobe shown posteriorly with the main placenta anteriorly.
Careful assessment for more than one succenturiate lobe in necessary. In this case 3 distinct placental lobes are seen.
Vascularity and vessels are seen coursing between each lobe.
In third trimester it is extremely common to observe calcification within the placenta however the clinical significance remains unclear.
It is very common to see venous lakes within the placenta. However caution must be exercised as they may be associated with partial mole, complete mole, hydatidiform mole, Placental mesenchymal dysplasia,Chorioangioma, Placental accreta spectrum,Confined placental mosaicism, and Simple trophoblastic cyst.
The assessment of the cervix is extremely important in determining the risk for early onset of labour .
The cervix is dynamic with uterine contractions or a full bladder altering the length and true status. Generally the cervix should be assessed transvaginally at the morphology stage routinely unless there is a risk of premature rupture of membranes . In this case a transperineal approach is recommended.
The most accurate method for measuring the cervical length is in its longitudinal axis transvaginally. This is done by placing a caliper at the internal os to the external os. Avoid using excessive pressure as this may obscure funnelling and elongate the cervix.
Assessment should be done over a 3-5 minute period obtaining 3 measurements. The shortest measurement should be documented.
The most accurate method of visualising the placental position in relation to the internal os is transvaginally. A measurement should be made and a low lying placental position should be followed up in third trimester.
Transabdominal imaging of the cervix and placental position are not as accurate as transvaginal assessment.
These images show the lower uterine segment with a contraction can mimic a low lying placenta.