Oct 19

Obstetrics

Obstetrics

Obstetric 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 a standard part of prenatal care, as it yields a variety of information regarding the health of the mother and of the fetus, the progress of the pregnancy, and further information on the baby.

Types

Traditional obstetric sonograms are done by placing a transducer 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.[1]

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.[1][2] Coincidentally, most miscarriages also happen by 7 weeks gestation. The rate of miscarriage, especially threatened miscarriage, drops significantly if normal heartbeat is detected.[3]

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 of gestation, the fetal age may be estimated using the biparietal diameter (the transverse diameter of the head), the head circumference, the length of the femur, the crown-heel length (head to heel), and other fetal parameters. 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

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