Placenta I

The placenta serves as a function to exchange oxygen and carbon dioxide between mother and fetus. Usually maternal and fetal blood don't mix (only exception is till delivery), but if they do, supsect Rh pathology. The placenta also functions to work as defense mechanism to prevent viruses such as AIDS or Hep B&C to pass through the fetus (nicotine can also pass through as well). The placenta usually develops in the early second trimester (week 13), and becomes well established in week 15, which the retroplacental complex can be seen. In ultrasound, the placenta appears homogenous and echogenic. Later on, it becomes more heterogenous due to the development of calcifications and maternal lakes (veins and arteries).

The placenta is composed of three areas: basal layer, substance, and chorionic plate. The location can be anterior, posterior, fundal, or cervical, and must be determined in SAG. If found in the cervical region, in which the placenta has obstructed the whole area, then conisder in mind "placenta previa". The thickness of placenta should not exceed 4cm, and if thick then suspect hydatid form molar pregnancy (abnormal placental tissue implanted in UT).

The placenta can be broken down into stages or grades. Essentially, there are a total of 3 grades (0-3). As the placenta progresses through these grades, it becomes more echogenic and inactive. In Grade 0, the placenta has a smooth chorionic plate with no subtle or mild indentations. Grade 0 placenta is homogenous with no calcifications and is very active (necessary to promote gas exchange within the fetus). In Grade 1 placenta, the chorionic plate exhibits sublte indentations with punctuate calcifications. This normal placenta can be seen after 34 weeks. In Grade 2 placenta, the chorionic plate begins to have mild indentations with the development of comma-shaped calcifications within the placental substance. This normal placenta will appear more echogenic and can be seen after 36 weeks. Lastly, Grade 3 placenta will exhibit major indentations thereby dividing the placenta into segments. Linear calcifications can be seen within the substance and is normal after 38 weeks. For images on placental grading, click here.

The thing to keep in mind is that as the pregancy progresses, the placenta is able to move due to uterine size. As the fetus enlarges, so does the uterus thus affecting the position of the placenta. With that in mind, the placenta can move to the cervical area thereby obstructing the internal os. When this happens, this is termed "placenta previa".

There are different types of placenta previa: complete, partial, marginal, or low lying. Complete placenta previa is when the placenta has completely obstructed the internal cervical os. Partial placenta previa is when the placental tissue blocks only a portion of the internal cervical os. Marginal placental previa occurs when the substance extends up to but not above the internal cervical os (near proximal end of Cx). Low lying placenta, or potential placenta, is when the substance is 1cm away from internal cervical os. It is usually the low lying placenta that causes placental previa thereby inducing vascular malformation within the uterus. Another cause is abnormal uterine enlargement, which can cause the placenta to move towards the proximal end of the cervix. Below is video clip of placenta previa with the addition of abruptio placenta and the consequences that can happen with the condition.