"It is easier to rule out placenta previa by viewing the placenta at or reaching the uterine fundus"
Determining placental site and diagnosing placental previa:
Always keep the transducer at right angle to the ground not to the abdomen.
First look in the uterine fundus
a. If the placenta is reaching the uterine fundus then it is not previa.
b.If it is not reaching the fundus, examine the lower uterine segment -only with a full urinary bladder-.and with the probe perpendicular to the ground, place the probe parallel to the midline suprapubic measure the distance between the lower edge of the placenta and the cervical internal os:
A- At 18 to 22 weeks anomaly scan the distance is interpretted as follows:
- if the distance is = or > than 1cm from internal os, no further scan is required as placenta previa is unlikely.
- If the distance from internal os is less than 1 cm or if less than 2.5cm of the placenta is covering the internal os then repeat the scan at 35 weeks or earlier in case of bleeding.
- If more than 2.5sm of the placental tissue is covering the internal os then this is likely to be placenta previa at term. Verify placental site at 35 weeks and plan for CS delivery at term.
B- At third trimester and late scans the distance is interpreted as follows:
- More than 2cm and less than 5 cm from internal os = Low lying placenta, usually the fetal presenting part is applied to the cervix – Most probably this will not affect delivery unless if it bleeds.*
- Less than 2 cm = Placenta previa marginalis, the presenting part is not likely to be applied to the cervix, plan for CS.*
- Covering the internal os = Placenta previa centralis patients needs CS, follow the protocol of major previa.
*Amarnath Bhide, Federico Prefumo, Jessica Moore, Brian Hollis, Basky Thilaganathan; Placental edge to internal os distance in the late third trimester, and mode of delivery in placenta praevia, BJOG: an International Journal of Obstetrics and Gynaecology, September 2003, Vol. 110, pp. 860–864
Site of Umbilical cord insertion; just a quick look. Once in a lifetime it may astonish you that you can not spot the site of cord insertion... at this time you should think of velamentous cord insertion and vasaprevia.
Eccentric cored insertion (left) and marginal cord insertion (right)
Placental grading: Just to have an idea about the placental age but truly it does not reflect placental function; you need to reply on other parameters to assess placental function. The useful thing is if you find a GIII aged placenta with many calcifications and infarctions early in pregnancy when it is not expected (e.g 32 weeks) then you have to keep close watch on placenta function parameters (see below).
Grade 0: Late 1st trimester-early 2nd trimester, homogenous moderate echogenicity, smooth chorionic plate without indentations
Grade 1: Mid 2nd trimester –early 3rd trimester (18-29 wks), mild indentations of chorionic plate, small, diffuse calcifications (hyperechoic) randomly dispersed in placenta
Grade 2: Late 3rd trimester (30 wks to delivery), larger
indentations along chorionic plate, and larger
Grade 3: 39 wks – post dates, Complete indentations of chorionic plate through to the basilar plate creating "cotyledons” (portions of placenta separated by the indentations), More calcifications with significant shadowing,may signify placental dysmaturity which can cause IUGR, associated with smoking, chronic hypertension, SLE, diabetes. If seen earlier than 36 weeks beware of placental abruption
Placental bed should as well be checked for signs suggestive of placental adherence (accreta, increta and percreta) and for placental separation.
Overall assessment of placental function; this is a collection of information during the scan rather than one single parameter, the following are parameters of good placental function:
1. Normal fetal growth
2. Normal fetal movement
3. Average amniotic fluid volume
4. Umbilical flow with low resistance index (< 0.8)
5. Reactive Non-stress test