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Step 1: First and early scans
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A. In the first scan start by touring the abdomen and pelvis swiftly with the scan, you will be amazed at some findings that would just go un-noticed (as renal cysts, hydronephrosis, gall bladder stones, the cropus luteum cyst etc.) do not say that you can not diagnose such problems since you are an obstetrician, with time your eyes will be familiar with the normal views and you later spot abnormalities and refer her to radiology department. Another good thing is the patients reaction to what you are doing, she will definitely appreciate that you are the first obstetrician to “have a look around” instead of just taking CRL and fetal heart.
B. The first thing to spot at 5 weeks on-wards is the gestational sac, it should be in the uterus to exclude ectopic pregnancy.
Do not forget to record the number of gestational sacs...!
C. Detecting fetal pole starting from around 6 weeks gestation: Look for a white dot in the sac this is the fetal pole. Absent fetal pole may be due to an early scan when still a sac is visualized but not a fetal pole or may be blighted ovum (anembryonic sac) which is diagnosed if MSD (mean sac diameter is > 2cm without fetal pole) or if re-scan after 2 weeks still can not detect fetal pole. Beside the fetal pole you will see the secondary yolk sac.
D. Detecting fetal pulsations starting from 6 to 8 weeks gestation: look for fetal heart to assure fetal life is positive.
Fetal pulations - 6 weeks gestation
E. Measuring fetal crown-rump length: crown-rump length (CRL) is the measurement from the top of the head (crown) to the bottom of the buttocks (rump). It is measured as the largest dimension of embryo, excluding the yolk sac and extremities. Gestational age estimation is most accurate by CRL measurement, it carries less than 5 days falacy while 2nd trimester BPD only carries 7 days falacy, third trimester scan is not accurate to estimate gestational age.
It has been reported that MSD (mean sac diameter) should exceed CRL by at least 5 mm (i.e. MSD - CRL = > 5mm) as a parameter of healthy pregnancy, patients with MSD - CRL = < 5mm are very prone to first trimester abortion despite a normal heart rate. Chromosomal anomalies particularly Trisomy 18 and triploidy are markedly associated with growth restriction i.e. decreased crown rump length.
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Step 2: Examining the fetus and 10 point anomaly scan checklist
At least 2 scans are essential for anomaly scan the first at 11-13 weeks gestation and rescan at 20 weeks: What do you want to check?
I- Scanning at 10-14 weeks to spot nuchal thickness and nasal bone; Nuchal thickness is measured 11 to 14 weeks, practically, NT less than 2 mm carries low risk of chromosomal anomalies. Many criteria are put forward to "accurately" measure NT with the fetus in sagittal section and a neutral position of the fetal head neither hyperflexed nor extended. The fetal image is enlarged to fill 75 of the screen, and the maximum thickness is measured, from leading edge to leading edge. You may not be able to fulfill all the criteria but the most important is not to be "fooled" by the fetal amnion.
Nasal bone when absent (flat face) may reflect chromosomal anomaly. I find this when associated with increased NT is an indication to perform chorionic villous sampling.
II- Anomaly scan at 20 weeks for level I and II ultrasound, it is useful to have 10 point check list, it is simple without rare syndromes and can be applied easily in obstetric clinic. Simply look for the following 10 scan points:
fetal nasal bone
1. Skull bone is seen all around without big gaps, and Biparietal diameter correlates with the gestational age and other fetal measures excludes most skull anomalies and hydrocephalus.
Line of falx cerebri is not shifted and almost structures on both sides are similar in the view excludes most brain anomalies.
2- The spine is seen 3 rows of rosettes without something bulging out
Spine - stomach and diaphragm
3- The chest is TS showing the heart not shifted to one side of the midline, this pretty much excludes diaphragmatic hernia.
Chest showing heart not shifted
4- 4 chambers of the heart are seen, and rhythmic regular heart beats are present. I move the probe till I can visualize the aortic arch and below the arch the rounded image of the pulmonary artery (If both vessels run parallel this may indicate transposition of great vessels)
4 chamber view of the heart
5. Stomach visualization and Diaphragm seen as a line separating chest from abdomen.
6. Fetal Abdomen is seen complete circle in Abdominal circumference (AC) view at level of umbilical cord insertion. AC at level of umbilical vein should correlate with gestational age and other fetal measures (more than 2 weeks lag needs level III ultrasound scan assessment)
AC with stomach visualized
7. Fetal urinary bladder seen as well as fetal kidneys. Full bladder and average amniotic fluid with no cysts occupying the site of the kidney indicates "functioning" kidneys and excludes fetal kidney anomalies.
Normal kidney and urinary bladder
8. Limbs and Feet seen oriented properly, this is very easy in early scans around 18 weeks onward and difficult as the fetus grows and gets “stuck” in the uterus. Femur length and humeral length should correlate with the gestational age and other measures.
fetal left lower limb
9- Fetal sex identification (awaited by most patients) can be detected as early as 14 weeks, is best viewed with both femurs seen in one view and search between thighs for external genital structures. Do not mention the gender if you do not see labia in a female fetus (do not diagnose female fetus by exclusion)
Male fetus as seen at 15 weeks
10. Fetal activity and behavior; check for:
a. Limb movement
b. Trunk movement
c. Breathing movement
d. Fine and coordinated movement: finger sucking, closing fist, eye lid movement, swallowing .. etc.
e. Notice fetal reaction to sound (when playing fetal heart for example) and fetal movement in response to moving the probe on the maternal abdomen.
Right hand fingers seen moving
Step 3: Fetal measurements and biometry
MSD; Mean sac diameter; The gestational sac is measured in three dimensions, and the average, the Mean Sac Diameter (MSD) used for estimating gestational age. It is useful between 5 and 8 menstrual weeks with accuracy of +/- 3 days .
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.
BPD and HC
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.
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 and
demonstrate normal fetal proportions particularly in the second half
of the pregnancy.
The abdominal circumference is measured at the level of the liver and stomach, including the left portal vein at the umbilical region
AC at the level of the left poral vein
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
Calculated 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.
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Step 4: Assessment of chorionic plate and placenta
2-Site of Umbilical cord insertion
4-Overall assessment of placental function
Placental site: At early ultrasound scans the screen can accommodate the whole uterus, with the mother’s urinary bladder full you can easily spot the uterine fundus; just make sure the chorionic plate (future placenta) is reaching the fundus of the uterus. DO NOT make this comment on placental site in early or late pregnancy with an empty urinary bladder. It is easier to rule out placenta previa by viewing the placenta at or reaching the uterine fundus. I mean try to assure that any part of the placenta is located at the upper turn of the uterine fundus, if it is there then it can not be placenta previa. Also when fetal head is applied to the cervix this pretty much excludes placenta previa. If the placenta is not in the upper segment then with the mother's urinary bladder full, measure the distance between the lower pole of the placenta and the cervical internal os. The distance in third trimester may be:
More than 5 cm = Low lying placenta
Less than 5 cm = Placenta previa marginalis
Covering the internal os = Placenta previa centralis
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.
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 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 IUGR, associated with smoking, chronic hypertension, SLE, diabetes. If seen earlier than 36 weeks beware of placental abruption
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
Placenta Previa centralis
Placenta curving with the fundus, this excludes placenta previa
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Step 5: Amniotic fluid
Amniotic fluid can be assessed as early as first trimester scans the amniotic fluid volume matters; it has been found that MSD (mean sac diameter) greater than CRL with less than 5 mm is associated with miscarriage. 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 volume may associate cases of spina bifida, fetal GIT obstruction but in most cases it is idiopathic.
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:
i. Normal 10 cm – 20 cm
ii. Below average 6 cm – 9 cm
iii. Oligohydramnios < 5 cm
iv. Above average 20 cm -24 cm
v. Polyhydramnios > 24 cm
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