Ultrasound in obstetrics
Dr.Zahraa’ Muhmmed JameelIt uses very high frequency sound waves of between 3.5 (abdominal u/s)and 7.0 (transvaginal u/s)megahertz emitted from transducer.(Audible sound from 20 to 20000 hertz)
Transducers can be trans abdominal or transvaginal.
The probe contain piezoelectric crystals which change electricity to sounds and then receive sounds reflection and change it into electricity and displayed on the screen
Is currently considered to be a safe, accurate, noninvasive and cost effective procedure. It has progressively become an indispensible obstetric tool and plays an important role in the care of every pregnant woman
Transabdominal Ultrasound
Transabdominal approach :
Lower frequency, lower resolution image
Curved linear transducer
Better visualized with full bladder
Can see coronal and sagittal views of organs and fetus
Transvaginal Ultrasound
Higher frequency, higher resolution image
Endocavitory probe
Better visualized with empty bladder
Can see sagittal or coronal view of uterus
If possible attempt trans abdominal before considering transvaginal to avoid more invasive procedure.
Useful in:
Early pregnancy
Examination of cervix later in pregnancy
Identifying lower edge of placenta
In women with significant amount s of abdominal adipose tissue
The main uses of ultrasonography are in the following areas:
The gestational sac can be visualized as early as 4-5 weeks of gestation
The yolk sac at about 5 weeks.
The embryo can be observed and measured by about 5-6weeks.
A visible heart beat can be visualized by 6 week
Confirm the site of the pregnancy is within the cavity of the uterus
Ectopic pregnancy is suspected if, +ve preg. test, no GS within the uterus, adnexal mass with or without fetal pole, or there is fluid in the pouch of douglas .
Missed miscarriage: fetus identified with an absent fetal heart(CRL >6mm + no fetal heart)
Blighted ovum :empty gestational sac in the absence fetal development(GS >20mm +no fetal pole)
2. Determination of gestational age and assessment of fetal size &growth.
a)The gestational sac measurement 5-6 week of gestation
b) The Crown-rump length (CRL) can be made 7-13 weeks+6 days
c) The Head circumference (HC)from 14-20 week of gestation
d) The Biparietal diameter (BPD) after 13 weeks
e) The Femur length (FL) after 14weeks
f) The Abdominal circumference (AC) It reflects more of fetal size and weight rather than age.
The weight of the fetus at any gestation can also be estimated with great accuracy using polynomial equations containing the BPD, FL, and AC.
Essentially, the earlier the measurement is made, the more accurate prediction (from early CRL ±5 days is preferred to BPD at 20 weeks ±7 days) & GA cannot be calculated by u/s after 20 weeks B/c larger range of normal values of BPD & HC around the mean.
In pregnancies at high risk of FGR, serial measurements are plotted on the normal reference range.
Growth pattern help to distinguish b/w symmetrical and asymmetrical FGR (BPD,AC)
Large HC compared to small AC in brain sparing effect (asymmetrical IUGR)
In diabetic preg, AC is large due to effect of insulin on fetal liver and fat stores
Cessation of growth is ominous sign of placental failure
3. Multiple pregnancies.
Determining the number of fetuses, the chorionicity, fetal presentations, evidence of growth retardation and fetal anomaly, the presence of placenta previa, and any suggestion of twin-to-twin transfusion.
Thickness of dividing membrane thin in monochorionic twin & thick in dichorionic twin
The optimal GA to determine chorionicity is at 9-10 week
Later in pregnancy, dichorionicity is determined by identification of 2 placental masses &2 different sex fetuses
No septum in monochorionic monoamniotic twin
Thin septum in monochorionic diamniotic twin Formed by 2 layers of amnion
Thick septum formed by 2 layers of amnion &2 layers of chorionLambda sign or twin peak sign(dichorionic diamniotic twin)
5-Placental localization
At 20 week scans, lower uterine has not yet formed & most low lying placenta appear to migrate upward as the lower uterine segment stretches in the late 2nd &3rd trimester
5% of women have a low lying placenta at 20 weeks, only 5% of this group will eventually be shown to have placenta previa
Transvaginal u/s can be used to define lower edge of placenta if not seen by abdominal probe
6- Amniotic fluid assessment
Amount of AF in the uterus is a guide of fetal wellbeing in the 3rd trimester
Oligohydramnios is reduction in AF volume
Polyhydramnios is increase in AF volume
2 ultrasonic measurement approaches give indication of AF volume these are : MVP &AFI
The Maximum vertical pool is measured after a general survey of uterine content (2-8)cm
The Amniotic fluid index is measured by dividing the uterus into 4 u/s quadrants , a vertical measurement is taken of the deepest cord free pool in each quadrant and the result are summated
The fetus has a role in the control of the volume of AF. It swallows AF, absorbs it in the gut & later excretes urine in the amniotic sac
Congenital abnormalities that impair swallow, for example anencephaly or oesophagial atresia, will result in increase of .AF
Congenital abnormalities that result in failure of urine production or passage, for example renal agenesis & posterior urethral valves, will result in reduced or absent AF.
IUGR can be associated with reduced AF b/c of reduced urine output.
7- Assessment of fetal wellbeing
A-Biophysical profile is along (30 minutes) ultrasound scan which observes fetal behavior, measure AF volume& include a CTG.
Fetuses spend 30% of their time asleep, during which they are not active, & do not exhibit breathing movements.
B-Doppler investigations
Doppler u/s can be used to assess placental function & identify evidence of blood flow redistribution in the fetus, which is a sign of hypoxia
For umbilical artery give information about placental health and function
Pulstality index or resistance index(RI) :a measure of the amount of diastolic flow to systolic flow
An infarcted placenta (HT) will have increase resistance to flow( low RI)
Absent or reverse diastolic flow in the UA has strong correlation with fetal distress & IUD
For fetal vessels (middle cerebral arteries , aorta, IVC & ductus venosus)
Increase diastolic flow in MCA in hypoxia
Increase velocity in MCA in anemia (Rhesus disease, twin-to- twin transfusion syndrome)
8- Measurements of cervical length
Can be assessed by transvaginal u/s.
9- Other uses:
Confirmation of IUD
Confirmation of fetal presentation in uncertain cases
Diagnosis of uterine & pelvic abnormalities during pregnancy, for example fibromyomata & ovarian cyst.
u/s & invasive procedures
Is used to guide invasive diagnostic procedures such as amniocenteses, chorionic villous sampling & cordocentesis.
Is used to guide invasive therapeutic procedures such as insertion of fetal bladder shunt & chest drain
Fetoscopy
Doppler ultrasound and the prediction of adverse pregnancy outcome
Uterine arteries Doppler in 20-24 week have been used to predict preg. Outcome.
may demonstrate markers of increase resistance to flow including diastolic notch in the waveform in the early diastole
This associated with adverse pregnancy outcomes as PE, IUGR & abruption.
Summary of the aims of obstetric ultrasound
The early preg. Scan (11-14 weeks)
To confirm fetal viability
To provide accurate estimation of GA
To diagnose multiple gestation, in particular to determine chorionicity
To identify markers which indicate an increased risk of fetal chromosomal abnormality such as down syndrome
To identify fetuses with gross structural abnormalities
The 20 week scan(18-22)weeks
To provide accurate estimation of GA if an early scan has not been performed
To carry out a detailed fetal anatomical survey to detect any fetal structural abnormalities or markers of chromosomal abnormality
To locate placenta and identify the 5% of women with a low-lying placenta for a repeat scan at 34 weeks to exclude placenta previa
To estimate AF
Also in some centers to:
To perform Doppler u/s examination of maternal uterine arteries to screen for adverse pregnancy outcome, for example :PE
To measure cervical length to assess risk of preterm deliveries
Ultrasound in the third trimester
To assess fetal growth
To assess fetal well-being