Obstetric ultrasonography

# Overview
Obstetric sonography (ultrasonography) is the application of medical ultrasonography to obstetrics, in which ultrasound is used to visualize the embryo or fetus 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 ultrasounds are done by placing a transducer (a probe that emits high frequency sound waves) on the abdomen of the pregnant woman.  One variant is a transvaginal ultrasound 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 ultrasound which detects the heartbeat of the fetus.  Doppler ultrasound 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.

# 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 diameter of the head) and the length of the femur (the longest bone in the body).  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 baby can usually be determined by ultrasound at any time after 16 weeks, often at the dating scan around 20 weeks into the pregnancy depending upon the quality of the sonographic machine and skill of the operator. This is also the best time to have an ultrasound done as most infants are the same size at this stage of development.  Depending on the skill of the sonographer, ultrasound may suffer from a high rate of false negatives and false positives.  This means care has to be taken in interpreting the accuracy of the scan.

# Abnormality Screening
In some countries, routine pregnancy ultrasound 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 are in fact normal.

# History
In 1962, after about two years of work, Joseph Holmes, William Wright, and Ralph Meyerdirk developed the first compound contact B-mode scanner.  Their work had been supported by U.S. Public Health Services and the University of Colorado. Wright and Meyerdirk left the University to form Physionic Engineering Inc., which launched the first commercial hand-held articulated arm compound contact B-mode scanner in 1963. This was the start of the most popular design in the history of ultrasound scanners.
Obstetric ultrasound has played a significant role in the development of diagnostic ultrasound technology in general.  Much of the technological advances in diagnostic ultrasound technology are due to the drive to create better obstetric ultrasound equipment.  Acuson Corporation's pioneering work on the development of Coherent Image Formation helped shape the development of diagnostic ultrasound equipment as a whole.

# Safety issues
Current evidence indicates that diagnostic ultrasound is safe for the unborn child, unlike radiographs, which employ ionizing 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 called for.
A 2006 study on mice exposed to ultrasound 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.