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Hundereds of carefully categorized obgyn illustrations, and real life ultrasound scan images and clips from clinical practice with desription, user comments lightbox..etc.
Late Scans (Third trimester)
After 28 weeks till delivery the value of doing serial scans is mainly for assessment of fetal growth, maturity, well being, and as EDD approaches it is important to consider the EFW (expected fetal weight), lie, position and presentation. As the fetal organs grow, it is now easier to have a better look at the fetal CNS, fetal heart, and kidneys.
3- Maturity and well-being
1-Morphology (Fetus, Placenta, Amniotic fluid):
1-Malformations and anomalies: some malformations can still be spotted, e.g. renal, some cardiac anomalies, vertebral column..etc.
2-As delivery is approaching it is important to determine the lie, presentation, position and fetal weight. Always document the position of the back, sometimes cord presentation, compound presentation are seen.
Placental site:In late pregnancy it is common to over-diagnose placenta previa. To correctly diagnose placental site follow the following steps:
1-Always keep the probe at right angle to the ground not to the abdomen, this is to visualize structures directly beneath the probe.
2-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 segmentonly with a full urinary bladder.
3-With a full urinary bladder and with the probe perpendicular to the ground, place the probe parallel to the midline suprapubic to examine the LUS and you should be able to see the cervix.
a. If the fetal head is applied to the cervix this pretty much excludes placenta previa.
b.If the placenta is seen in the LUS below the presenting part of the fetus then measure the distance between the lower pole of the placenta and the cervical internal os. The distance in third trimester may be:
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. It is more difficult to spot the site of UC insertion as pregnancy advances.
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 finding is to find a GIII aged placenta with many calcifications and infarctions early in pregnancy when it is not expected (e.g 32 weeks) anticipate fetal growth restriction and /or placental abruption (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 calcifications.
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 FGR, associated with smoking, chronic hypertension, SLE, diabetes. If seen earlier than 36 weeks beware of placental abruption
c. Amniotic fluid:
Amniotic fluid index is the objective measure of the subjective assessment of AFV, measure the vertical depth of the largest pocket in every quadrant of the 4 uterine quadrants. The sum of the 4 would be considered:
Depending on a single amniotic fluid pocket measurement Oligohydramnios is diagnosed if no fluid pocket found with vertical depth >2cm (some authors consider 1cm) and polyhydramnios is diagnosed if there is an amniotic fluid pocket > 8cm.
Significantly diminished amniotic fluid volume (AFV)in second trimester almost always denotes fetal renal dysgenesis, while if normal in second trimester then starts to decrease in the third trimester may associate placental insufficiency and fetal growth restriction.Increased amniotic fluid volumemay associate cases of spina bifida, fetal GIT obstruction but in most cases it is idiopathic.
2- Measurements and fetal Growth
BPD; Biparietal diameter; The BPD should be measured as early as possible after 13 weeks for accurate dating. It is measured from leading edge to leading edge of the skull in the plane where:
1. falx cerebri in the midline,
2. the thalami symmetrically positioned on either side of the falx,
3. visualization of the Septum Pellucidum at one third the frontooccipital distance,
4. the head should look oval at that level.
A wrong measurment plane can produce errors up to 20mm.
HC; Head circumference; useful measurement to avoid considering a flat head as microcephaly and used as well to calculate fetal weight, other than this I find it an inaccurate measure to estimate gestational age
AC; Abdominal circumference; Not useful for dating, it gives an estimate of the weight and size of the fetus and is important when doing serial ultrasounds to monitor fetal growth (in FGR the liver is affected and becomes smaller leading to decreased AC) and to demonstrate normal fetal proportions particularly in the second half of the pregnancy.
FL; Femur length; Measured from 14 weeks onwards, and in my opinion it remains accurate at late gestation. Measurment of the FL should be taken when both ends of the femur bone are blunt and the femur is almost parallel to the probe (The extension to the greater trochanter and the head of femur should not be included) an angle of over 30 degrees to the horizontal makes the measure inaccurate. The FL of dwarfs are at least nearly 4-5 weeks behind the dates. Abnormality in the shape of the limb will also be present however they are difficult to assess as the lateral surface of the femur is almost always straight and the medial surface is almost always curved
HL; Humeral length; The long bones are measured with the bone across the beam axis. blunt ends indicates that the image plane is on the longest axis and is the optimal measurement plane. Shortened humerus length has a greater sensitivity than femur length in cases of trisomy 21
EFW; Expected fetal weight
N.B: Late scan if done in third trimester should comment on fetal lie, position of the back, amniotic fluid volume, placental grade and EFW. It is a lot of fun to compare last scan EFW with the actual birth weight.
HC/AC between 20 and 36 weeks drops steadily from almost 1.2 to 1. It will be increased in cases of asymmetrical FGR and will remain within normal ratio in cases of symmetrical FGR.
1. Normal fetal growth
2. Normal fetal movement
3. Average amniotic fluid volume
4- Placental grade II
4. Normal Umbilical vessels Doppler flow assessment
5. Reactive Non-stress test