Obstetrical Ultrasound
|
Obstetric sonography (ultrasonography) is the application of medical ultrasonography to obstetrics, in which sonography is used to visualize the embryo or foetus in its mother’s uterus (womb). The procedure is often a standard part of prenatal care, as it yields a variety of information regarding the health of the mother and of the fetus, as well as regarding the progress of the pregnancy.
Types Traditional obstetric sonograms are done by placing a transducer (a device that converts one type of energy into another) on the abdomen of the pregnant woman. One variant, a transvaginal sonography, is done with a probe placed in the woman’s vagina. Transvaginal scans usually provide clearer pictures during early pregnancy and in obese women. Also used is Doppler sonography which detects the heartbeat of the fetus. Doppler sonography can be used to evaluate the pulsations in the fetal heart and bloods vessels for signs of abnormalities. Early pregnancy The gestational sac can sometimes be visualized as early as four and a half weeks of gestation (approximately two and a half weeks after ovulation) and the yolk sac at about five weeks gestation. The embryo can be observed and measured by about five and a half weeks. The heartbeat may be seen as early as 6 weeks, and is usually visible by 7 weeks gestation. Coincidentally, most miscarriages also happen by 7 weeks gestation. The rate of miscarriage, especially threatened miscarriage, drops significantly if normal heartbeat is detected. Dating and growth monitoring Gestational age is usually determined by the date of the woman’s last menstrual period, and assuming ovulation occurred on day fourteen of the menstrual cycle. Sometimes a woman may be uncertain of the date of her last menstrual period, or there may be reason to suspect ovulation occurred significantly earlier or later than the fourteenth day of her cycle. Ultrasound scans offer an alternative method of estimating gestational age. The most accurate measurement for dating is the crown-rump length of the fetus, which can be done between 7 and 13 weeks of gestation. After 13 weeks gestation, the fetal age may be estimated by the biparietal diameter (the transverse diameter of the head), the head circumference, the length of the femur (the longest bone in the body), and the many more fetal parameters that have been measured and correlated with age over the last 30 years. Dating is more accurate when done earlier in the pregnancy; if a later scan gives a different estimate of gestational age, the estimated age is not normally changed but rather it is assumed the fetus is not growing at the expected rate. Not useful for dating, the abdominal circumference of the fetus may also be measured. This gives an estimate of the weight and size of the fetus and is important when doing serial ultrasounds to monitor fetal growth. Fetal sex determination The sex of the fetus may be determined by ultrasound as early as 11 weeks gestation. The accuracy is relatively imprecise when attempt early. After 13 weeks gestation, a high accuracy of between 99% to 100% is possible if there is no malformed external genitalia. Influencing Factors The accuracy of fetal sex determination depends on:
Ultrasonography of the cervix Obstetric sonography has become useful in the assessment of the cervix in women at risk for premature birth. A short cervix preterm is undesirable: At 24 weeks gestation a cervix length of less than 25 mm defines a risk group for preterm birth, further, the shorter the cervix the greater the risk. It also has been helpful to use ultrasonography in women with preterm contractions, as those whose cervix length exceed 30 mm are unlikely to deliver within the next week. Abnormality screening In some countries, routine pregnancy sonographic scans are performed to detect developmental defects before birth. This includes checking the status of the limbs and vital organs, as well as (sometimes) specific tests for abnormalities. Some abnormalities detected by ultrasound can be addressed by medical treatment in utero or by perinatal care, though indications of other abnormalities can lead to a decision regarding abortion. Perhaps the most common such test uses a measurement of the nuchal translucency thickness (“NT-test”, or “Nuchal Scan”). Although 91% of fetuses affected by Down syndrome exhibit this defect, 5% of fetuses flagged by the test do not have Down syndrome. Ultrasound may also detect fetal organ anomaly. Usually scans for this type of detection are done around 18 to 20 weeks of gestational age. Safety issues Current evidence indicates that diagnostic ultrasound is safe for the unborn child, unlike radiographs, which employionizing radiation. However, no randomized controlled trials have been undertaken to test the safety of the technology, and thus ultrasound procedures are generally not done repeatedly unless medically indicated. A 2006 study on genetically modified mice exposed to ultrasound (5-240 minutes a day) showed neurological changes in the exposed fetuses. Some of the rodent brain cells failed to migrate to their proper position and remained scattered in incorrect parts of the brain. It has been shown that Low Intensity Pulsed Ultrasound does have a localized effect on growth in human beings. The 1985 maximum power allowed by the U.S. Food and Drug Administration (FDA) of 180 milliwatts per square cm is well under the levels used in therapeutic ultrasound, but still higher than the 30-80 milliwatts per square cm range of the Statison V veterinary LIPUS device. LIPUS has been shown to affect tissue growth in as little as 20 minutes of time with repeated daily applications. Adding to the similarity, LIPUS and medical ultrasound both operate in the 1 to 10 MHz range. While the benefits of medical ultrasound outweigh any risks, vanity uses such as making 3D ultrasound movies without a doctor’s order present a possibly unnecessary, but unknown risk to a developing fetus. The FDA discourages its use for non-medical purposes such as fetal keepsake videos and photos, even though it is the same technology used in hospitals. The demand for keepsake ultrasound products in medical environments has prompted commercial solutions such as self-serve software that allows the patient to create a “keepsake” from the ultrasound imagery recorded during a medical ultrasound procedure. |
