What is women's ultrasound scanning?

Currently we use equipment known as real-time scanners, which produce a continuous picture of the moving fetus on a monitor screen. Very high frequency sound waves are used to produce these images. They are well above the range of human hearing. They are emitted from a transducer which is placed on the maternal abdomen, and is moved to view the contents of the uterus. Movements such as fetal heart beat can be assessed and measurements can be made on the images displayed on the screen. Such measurements allow the accurate assessment of gestational age, fetal size and growth.

What is ultrasound used for in pregnancy?

Ultrasound is considered to be a safe, non-invasive, accurate and cost-effective way to observe in the fetus. It has become a useful tool in obstetrics and now plays a role in the care of every pregnant woman. The main uses of ultrasonography are:

Diagnosis and assessment of early pregnancy.

The gestational sac can be visualized as early as the four and a half weeks of gestation and the yolk sac at about five weeks.

Threatened miscarriage

The viability of the fetus can be documented in the presence of vaginal bleeding in early pregnancy. Fetal heart motion is usually detectable by 7 weeks. If this is observed, the probability of a continued pregnancy is greater than 97 percent. Missed abortion and blighted ovum will usually give the picture of a deformed gestational sac and absence of fetal poles or heart beat.

Ultrasonography is also indispensable in the early diagnosis of ectopic pregnancies and molar pregnancies.

Determination of gestational age and assessment of fetal size.

Fetal body measurements reflect the gestational age of the fetus. This is particularly true in early gestation. In patients with uncertain last menstrual periods, such measurements must be made as early as possible in pregnancy to arrive at a correct dating for the patient.

The following measurements are usually made:

a) The Crown-Rump Length (CRL)

This measurement can be made between 7 to 13 weeks and gives very accurate estimation of the gestational age. Dating with the CRL can be within 3-4 days of the last menstrual period. Chart

b) The Biparietal Diameter (BPD)

The diameter between the 2 sides of the head. This is measured after 13 weeks. It increases from about 2.4 cm at 13 weeks to about 9.5 cm at term. Different babies of the same weight can have different head size, therefore dating in the later part of pregnancy is generally considered unreliable. Chart

c) The Femur Length (FL)

Measures the longest bone in the body and reflects the longitudinal growth of the fetus. Its usefulness is similar to the BPD. It increases from about 1.5 cm at 14 weeks to about 7.8 cm at term. Chart

d) The Abdominal Circumference (AC)

The single most important measurement to make in late pregnancy. It reflects more of fetal size and weight rather than age. Serial measurements are useful in monitoring growth of the fetus. Chart

The weight of the fetus at any gestation can also be estimated. Lookup charts are available. For example, a BPD of 9.0 cm and an AC of 30.0 cm will give a weight estimate of roughly 2.85 kg.

Placental Localization

Ultrasonography is very helpful in the diagnosis of placenta previa, and other placental abnormalities.

Multiple pregnancies

Ultrasonography is valuable in determining the number of fetuses and their presentations.

Amniotic Fluid Assessment

Hydramnios and oligohydramnios, excessive or decreased amount of amniotic fluid can be detected by ultrasound.

Fetal malformations

Some structural abnormalities in the fetus can be detected by an ultrasound scan. Ultrasound can also assist in other diagnostic procedures in prenatal diagnosis such as amniocentesis

Transvaginal Scan

Ultrasound scanning can be done with the probe placed in the vagina of the patient.This method usually provides better images in patients who are not pregnant or are in the early stages of pregnancy. Fetal cardiac pulsation can be observed as early as 6 weeks of gestation. Vaginal scans are very helpful in the early diagnosis of ectopic pregnancies.

Is ultrasound safe?

It has been over 35 years since ultrasound was first introduced into obstetrics and gynecology. Unlike X-rays, ionizing irradiation is not present and harmful fetal effects associated with such irradiation have not been found to occur. Harmful effects in cells of experimental animals or humans have not been demonstrated in the large amount of studies that have so far appeared in the medical literature dealing with the use of diagnostic ultrasound in the clinical setting.

Ultrasound in Gynecology

Ultrasound can be very helpful in diagnosis a whole host of gynecologic disorders. It is a relatively non-invasive method of viewing the reproductive organs in the pelvis to determine the nature and extent of structural abnormalities present. Many gynecologic disorders do not show early symptoms and ultrasound can often diagnose problems well before a patient may know a problem exists.

The Uterus

Uterine size and shape can be determined. Common problems such as uterine fibroid tumors can be localized, measured and followed over time to help determine their significance. Fibroids can cause pain, bleeding and interfere with fertility. Endometrial lining thickness can be measured in patients who experience abnormal uterine bleeding to help rule out cancer as a cause of their bleeding.

The Ovaries

Ovarian size can be determined. Cysts inside and outside of the ovary can be measured and their characteristics determined. Ovulatory function of the ovaries can also be assessed by serial ultrasound exams.

The Fallopian Tubes

Ectopic pregnancy is a common problem. Ectopic pregnancy occurs when implantation of the embryo is in the fallopian tube instead of in the uterus. Ultrasound can help visualize ectopic pregnancy and other fallopian tube abnormalities.

The Pelvis

Fluid in the pelvic can be a cause of pelvic pain. It can occur for several reasons such as ruptured ovarian cysts, ovulation and bleeding from the ovary or fallopian tubes. The presence of fluid in the pelvis can be determined by ultrasound.

In summary, ultrasound is now considered to be an essential tool in the evaluation of many different problems that occur in women and it has the appealing aspects of being relatively non-invasive and cost effective.

Obstetric and Gynecologic Ultrasound Links

American Institute of Ultrasound in Medicine
Journal of Ultrasound in Medicine
Ultrasound in Obstetrics and Gynecology
Obstetric Ultrasound - Dr. Joseph Woo
Sheathing Technologies  

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