Obstetric ultrasonography, or prenatal ultrasound, is the use of medical ultrasound in pregnancy, in which sound waves are used to create real-time visual images of the developing embryo or fetus in the uterus (womb). The procedure is a standard part of prenatal care in many countries, as it can provide a variety of information about the health of the mother, the timing and progress of the pregnancy, and the health and development of the embryo or fetus. The International Society of Ultrasound in Obstetrics and Gynecology (ISUOG) recommends that pregnant women be offered routine obstetric ultrasounds between 18 weeks' and 24 weeks' gestational age (the anatomy scan) in order to confirm pregnancy dating, to measure the fetus so that growth abnormalities can be recognized quickly later in pregnancy, and to assess for congenital malformations and multiple pregnancies (twins, etc.). Additionally, the ISUOG recommends that pregnant patients who desire genetic testing have obstetric ultrasounds between 11 weeks' and 13 weeks 6 days' gestational age in countries with resources to perform them (the nuchal scan). Performing an ultrasound at this early stage of pregnancy can more accurately confirm the timing of the pregnancy, and can also assess for multiple fetuses and major congenital abnormalities at an earlier stage. Research shows that routine obstetric ultrasound before 24 weeks' gestational age can significantly reduce the risk of failing to recognize multiple gestations and can improve pregnancy dating to reduce the risk of labor induction for post-dates pregnancy. There is no difference, however, in perinatal death or poor outcomes for infants.
Terminology
Below are useful terms on ultrasound:
Echogenic – giving rise to reflections (echoes) of ultrasound waves Hyperechoic – more echogenic (brighter) than normal Hypoechoic – less echogenic (darker) than normal Isoechoic – the same echogenicity as another tissue Transvaginal ultrasonography – Ultrasound is performed through the vagina Transabdominal ultrasonography – Ultrasound is performed across the abdominal wall or through the abdominal cavity In normal state, each body tissue type, such as liver, spleen or kidney, has a unique echogenicity. Fortunately, gestational sac, yolk sac and embryo are surrounded by hyperechoic (brighter) body tissues.
Types
Traditional obstetric sonograms are done by placing a transducer on the abdomen of the pregnant woman. One variant, 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.
3D ultrasound
Modern 3D ultrasound images provide greater detail for prenatal diagnosis than the older 2D ultrasound technology. While 3D is popular with parents desiring a prenatal photograph as a keepsake, both 2D and 3D are discouraged by the FDA for non-medical use, but there are no definitive studies linking ultrasound to any adverse medical effects. The following 3D ultrasound images were taken at different stages of pregnancy:
Medical uses
Early pregnancy A gestational sac can be reliably seen on transvaginal ultrasound by 5 weeks' gestational age (approximately 3 weeks after ovulation). The embryo should be seen by the time the gestational sac measures 25 mm, about five and a half weeks. The heartbeat is usually seen on transvaginal ultrasound by the time the embryo measures 5 mm, but may not be visible until the embryo reaches 19 mm, around 7 weeks' gestational age. Coincidentally, most miscarriages also happen by 7 weeks' gestation. The rate of miscarriage, especially threatened miscarriage, drops significantly after normal heartbeat is detected, and after 13 weeks.
First trimester In the first trimester, a standard ultrasound examination typically includes:
Gestational sac size, location, and number Identification of the embryo and/or yolk sac Measurement of fetal length (known as the crown-rump length) Fetal number, including number of amnionic sacs and chorionic sacs for multiple gestations Embryonic/fetal cardiac activity Assessment of embryonic/fetal anatomy appropriate for the first trimester Evaluation of the maternal uterus, tubes, ovaries, and surrounding structures Evaluation of the fetal nuchal fold, with consideration of fetal nuchal translucency assessment
Second and third trimester
The standard mid-trimester ultrasound survey relies heavily on clear visualization of fetal anatomy, including structural monitoring of limbs, extremities, and head parameters to evaluate development and detect anomalies. In the second trimester, a standard ultrasound exam typically includes:[12]
Fetal number, including number of amnionic sacs and chorionic sacs for multiple gestations Fetal cardiac activity Fetal position relative to the uterus and cervix Location and appearance of the placenta, including site of umbilical cord insertion when possible Amniotic fluid volume Gestational age assessment Fetal weight estimation Fetal anatomical survey Evaluation of the maternal uterus, tubes, ovaries, and surrounding structures when appropriate
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 14 weeks of gestation. After 14 weeks of gestation, the fetal age may be estimated using the head circumference with or without the length of the femur. 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. 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 discernment
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