Obstetric Ultrasound

Pregnancy Ultrasound Scan

Its inception in the medical field, it has become a very useful diagnostic tool in Obstetrics and applied very frequently.

Knowledge about various obstetric ultrasound measurements is important during transcription to avoid gross mistakes, which would make the meaning otherwise. Therefore, it is imperative to know what is obstetrical ultrasound and what it is used for.

The currently used equipment for such a scan are called as real-time scanners with the ability to provide continuous picture of a moving fetus on a monitor screen. Very high frequency sound waves of between 3.5 to 7.0 megahertz are generally used for this purpose. These waves are emitted from a transducer, which is placed in contact with the maternal abdomen and is moved to the particular part of the uterus. These frequencies when reflected back from the fetal surface produce a typical sonographic image, which can be read and categorized with various computer software's


Spirometry measures how much air a patient can inhale and exhale and how fast they can exhale. It is an objective measurement used for diagnostic evaluation.


With this ability, fetal movements such as heart beat and malformations in the fetus can be assessed and accurate measurements can be obtained from the images displayed.


The most important thing for a successful scan is the fullness of a bladder and hence all pregnant females are always requested to drink a lot of water before the scan.


Ultrasound in pregnancy

Ultrasound scan is currently considered to be the most safe, reliable, non-invasive, accurate and cost-effective diagnostic investigation for the fetal well being. Because of these qualities, it has become an indispensable obstetric tool in assessing the health of the baby and the mother.


Obstetrical ultrasound can be useful for

  1. Diagnosis of fetal malformation - Many structural abnormalities in the fetus can be diagnosed by an ultrasound scan usually by 20 weeks. Some anomalies like hydrocephalus, anencephaly, myelomeningocele, achondroplasia, spina bifida, exomphalos, and fetal hydrops can be accurately diagnosed so early in fetal life.
  2. Diagnosis and confirmation of early pregnancy - The most important part of ultrasound is confirm the diagnosis of pregnancy and the gestational sac can be visualized as early as four and half weeks of gestation. Therefore, further precautions related to maternal health or desires can be effectively looked after.
  3. Vaginal bleeding - Any abnormality to the fetus due to uterine bleeding can be documented but tabulating the heartbeat as early as 6 weeks.
  4. Missed abortion and blighted ovum can be diagnosed very early part of fetal life because of typical sonographic pictures and with deformed gestational sac and absence of fetal poles or heart beat.
  5. Multiple pregnancies - Can be diagnosed in early part of gestation.
  6. Determination of gestational age and assessment of fetal size - In patients with uncertain last menstrual periods, ultrasound can usually accurately measure the estimated date of conception and estimated date of delivery.
  7. Placental localization - The diagnosis of Placenta previa, which almost complicates each pregnancy, can be diagnosed very early through the ultrasound.
  8. Hydramnios and Oligohydramnios - Excessive or decreased amount of amniotic fluid can be depicted by ultrasound.

Obstetrical Ultrasound Measurements

These are done with the help of lot of measurements of the fetus and then tabulating them to get the desired result.
The most common measurements that are generally taken are:
  • Crown-rump length (CRL) - This measurement can be made between 7 to 13 weeks and gives very accurate estimation of the gestational age.
  • Biparietal diameter (BPD) - Is the diameter between the two sides of the head. This is measured after 13 weeks.
  • Femur length (FL) - Measures the longest bone in the body and reflects the longitudinal growth of the fetus.
  • Abdominal circumference (AC) - Reflects the fetal size and weight. Serial measurements of AC are useful in monitoring growth of the fetus.
  • Weight of the fetus - This can be ascertained with great accuracy using polynomial equations containing BPD, FL, and AC.
  • Head Circumference (HC) - The head circumference is used similar to the BPD for determination of age but it is little technically difficult to make and carries a high degree of error.
  • Ratios - Ratios of the Head circumference to the abdominal circumference (HC/AC ratio) and of the Femur length to abdominal circumference (FL/AC ratio) may often be used to assess fetal growth.
  • Gestational sac diameter (GS) - This is not so reliable but can give useful information.
  • Yolk sac diameter (YS) - This is not so reliable but can give useful information.

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