ULTRASOUND IN OBSTETRICS – ULTRASOUND WORKING, USE OF ULTRASOUND IN OBSTETRICS, TRANSVAGINAL ULTRASONOGRAPHY, DOPPLER ULTRASOUND AND MIDWIFE’S RESPONSIBILITY REGARDING PRENATAL SCREENING (Maternal and Child Health Nursing)
The ultrasound is a sound wave beyond the audible range of frequency greater than 2 MHz cycles per second). The commonly used frequency range in obstetrics is 3.5-5 MHz. SONAR stands for “Sound, Navigation and Ranging”. In clinical practice, two main varieties of ultrasound (sound that is produced at a very high pitch) are used depending upon whether the reflected waves give audible or visual signals
- The apparatus, which interprets the audible signals – doptone and sonicaid are easy to carry and simple to use even with batteries. It can detect fetal heartbeats are early as 10 week of gestation
- The apparatus for interpretation of visible signals – the sonar system, which is a much more sophisticated and bulky apparatus, issued in three forms
The A-scan, that gives a one-dimensional picture
The B-scan, that gives a composite two-dimensional picture
The real-time scanner that depicts movements – to display cardiac and breathing activity
HOW DOES ULTRASOUND WORK?
Scanners are used to produce static pictures. The picture is built up as a single crystal transducer (a thin disk to which a wire is attached) is moved backward and forward across the area scanned. When the transducer is placed on the body and as it encounters a structure, a fraction of that sound is reflected back. The echo is detected electronically and transmitted on to the screen as dots. The amount of sound from the each organ varies according to the type of tissue encountered:
- Strong echoes give bright dots, e.g. bone
- Weaker echoes give various shades of gray according to their strength
- Fluid-filled areas cause no reflexion and give rise to a black image
The real-time scanners are so called because it produces a moving picture on the screen as opposed to scanner that gives static picture. The real-time scanner can have several types of transducers attached to it, which are interchangeable and are used according to the type of image needed and the part of the anatomy to be examined. Types of transducers in common use include the linear array, the curved linear array, the sector and the vaginal probe. Instead of a single crystal, all these types of transducers have many crystals that fire off electrical energy and collect the echoes very rapidly, thus producing the moving picture.
USE OF ULTRASOUND IN OBSTETRICS
Sonography is a noninvasive procedure and has been proved safe to the conceptus, even with repeated exposures at any stage of pregnancy. Routine sonography in early months is used for:
- Diagnosis of pregnancy: detects gestational ring at 5th week, fetal poles and gestational sac at 6th week, cardiac pulsation at 7th week and embryonic movements at 8th week of gestation
- Detection of abnormal conceptus prior to clinical manifestations, and fetal malformations
- Accurate determination of gestational age is possible, which is helpful later in pregnancy when IUGR is suspected. For this, crown-rump length (CRL) at 10-11 weeks gives the best predictive value
- Diagnosis of twins can be made early in pregnancy for effective management
- To diagnose unsuspected placenta previa: because of the possibility of placental migration to the upper segment, repeat scanning should be performed later-around 34th week
Selective sonography is done when indicated at any time during pregnancy for the following reasons:
- To determine the maturity of the fetus: crown-rump length (CRL), biparietal diameter (BDP) and femur length (FL) are the measurements of choice for assessment of gestational age. Determination of the maturity is important in cases of:
Uncertainty gestational age
Discrepancy between amenorrhea and uterine size
Prior to elective induction of postmaturity or elective cesarean section
Suspicion of fetal and/or placental abnormalities such as:
Suspected ectopic pregnancy
Blighted ovum (empty sac)
Incomplete abortion
Hydatidiform mole
Localization of placenta as in placenta previa
Abruptio placentae
Intrauterine growth retardation
Intrauterine death
Malpresentations, such as breech, transverse or face
Structural defects, such as neural tube defects, absent or abnormal limbs
Defects of gastrointestinal and urinary system and heart defects
- Prior to invasive procedures such as chorion villus biopsy, amniocentesis, cordocentesis, photocopy and intrauterine fetal therapy
- As a part of antepartum or intrapartum fetal surveillance a biophysical profile
- Integrity of a previous cesarean scar – a weak scar or placental implantation over the scar can be detected
- Postpartum period
Secondary PPH
Retained placental bits
Subinvolution due to fibromyoma
- Neonatal head screening to diagnose:
Intraventricular hemorrhage
Hydrocephalus
TRANSVAGINAL ULTRASONOGRAPHY
Transvaginal ultrasonography (TUS) is usually done during the first trimester of pregnancy. As the transducer is closer to the object, the images are of enhanced quality. A full bladder is not required. Transvaginal sonography is superior to transabdominal sonography in diagnosing placenta previa
DOPPLER ULTRASOUND (AUDIBLE SIGNAL)
Ultrasound transmitted into the body in a narrow beam is transferred back at the same frequency when the object is still. When moving, there is a change in frequency known as the Doppler shift. The frequency increases or decreases according to whether the movement is toward or away from the source of energy
It is used for monitoring the fetal heart rate, which can be picked up as early as 10th week of gestation. Uterine and fetal blood flow can be assessed and the fetus at risk of compromise could be identified. Leg vein thrombosis can be diagnosed by noting the absence of hissing sound of blood flow through the veins
MIDWIFE’S RESPONSIBILITY REGARDING PRENATAL SCREENING
Midwives take care of pregnant women in different stages of their pregnancy. They often need to involve themselves in preparing and counseling women through the process. When giving information about the screening tests available or prescribed, it is important to include information regarding;
- Why the test is offered
- What the test involves and
- When and how the results will be given
In order to advise women regarding the tests that are available, the midwife needs to keep up-to-date with current technological advances
Information given to women should also include how the woman needs to prepare herself, e.g. by attending with a full bladder for a scan
