Ultrasound scanning is one of the most commonly used investigation in pregnancy.In this article I will try to summarize the use of ultrasound in pregnancy.

Ultrasound as the name suggests is the use of high frequency sound waves. The principle behind all ultrasound machines is the reflection of sound waves.Ultrasound was first developed for industrial use and for use in Ships to view the ocean floor so that they can avoid rocks etc.

Ultrasound scanning was first introduced to medicine in the 1950′s.

Ultrasound scanners in use today are real time scanners. In real time scanners the operator will be able to see live scans of the fetus, which is very useful so that he can move the probe to get the best possible view.

Many people think that the use of ultrasound in medicine is restricted to pregnancy, it is not so.Ultrasound scanning is used for many purposes in medicine. Modern ultrasound equipment for medical use uses very high frequency sound waves of 3.5 to 7.0 megahertz Some other examples of the use of ultrasound in medicine include echocardiography (Ultrasound of the heart), bladder scanning, scanning for renal and gallebladder stones. In this article we will restrict ourselves to the use of ultrasound in pregnancy.

Types of Ultrasound Scanning

1) Abdominal ultrasound- Also known as conventional ultrasound, is the commonest type of ultrasound used in pregnancy.Here the ultrasound probe is placed over the anterior abdominal wall. It requires less privacy than than the trans-vaginal ultrasound scan. Abdominal ultrasound requires that the woman empties her bladder.

2)Transvaginal ultrasound scan- The transvaginal ultrasound scan requires a special probe. The probe is placed in the vagina. This procedure requires adequate consent and privacy. To protect women from the transmission of sexually transmitted diseases via the probe, a male condom is pulled over the probe before insertion into the vagina.

The transvaginal method is the preferred method in early pregnancy,because it provides a better image of the uterus, tubes and the ovary in early pregnancy.

The transvaginal ultrasound is extremely useful in the diagnosis of ectopic pregnancy. Some ongenital abnormalities of the fetus can also be easily detected via this method.

3) Doppler ultrasound-Doppler ultrasound has been in use for a long time in fetal heart rate detectors.

Modern doppler ultrasound machines, enable the measurement of blood flow in fetal blood vessels(Flow velocity waveforms). A diminished blood flow in the diastolic phase is an indication of fetal compromise. The fetal blood vessels commonly studied are the middle cerebral artery, aorta and uterine arcuate arteries.

2d or 3d Ultrasound

In most developed countries, three dimensional  ultrasound scanning is readily available. It is give a better view of the fetus compared to the older generation 2d ultrasound.The three dimensional ultrasound is especially important in the diagnosis of congenital anomalies.

It has also become fashionable for parents to see the facial features of the baby, some clinics give a recorded form of the 3d ultrasound to the parents.

Uses of Ultrasound in Pregnancy:

i)Diagnosis of pregnancy

ii)Estimation of gestational age.

iii)Diagnosis of multiple pregnancy.

iv)Diagnosis of polyhydramnios and oligohydramnios- excessive or decreased amounts of liquor(Amniotic fluid).

v)Placental localization

vi)Fetal malformations/abnormalities

vii)Fetal echocardiography

viii)Confirmation of intrauterine death.

ix) Confirmation of fetal presentation.

x)Biophysical profile

xi)Diagnosis of uterine and pelvic abnormalities during pregnancy such as fibromyomata and ovarian cysts.

Some of the uses mentioned above are elaborated below:

Diagnosis, Confirmation and Assessment of Pregnancy

The gestation sac can be easily visualized around 4-5 weeks of gestation. The fetal heart beat can also be recognized around 5-6 weeks of pregnancy.

In the case of bleeding and pain, transvaginal ultrasound scan can confirm intrauterine pregnancy- at 4 weeks +3days for a regular 28day cycle or ectopic pregnancy and the viability of the fetus. Please note that ectopic pregnancy can present without bleeding.

Transvaginal ultrasound detects pregnancy and fetal heart sounds one week before the abdominal ultrasound scan. Vaginal ultrasound can detect early pregnancy by the 5th week which will appear as a small gestational sac. Abdominal scan can usually detect the pregnancy by the 6th week of gestation.One can also determine the fetal cardiac pulsation by vaginal ultrasound by the 5th week of gestation. Ultrasound can differentiate a live fetus from a dead fetus. It can also be used for adnexial and uterine masses. Unfortunately the interpretation of an ultrasound scan requires a lot of experience.

Determination of Gestational Age by Ultrasound.

The ultrasound scan is very useful in determining the gestational age of the fetus. In uncertain dates the determination of gestational age is very useful. A dating scan should be done around the 20th week.

The ultrasound scan is also very useful in the latter part of the pregnancy, where it is important in the diagnosis of intrauterine growth retardation.

Measurements that can be done by ultrasound scan to determine gestational age are:

i) Crown-rump length (between 7 to 13 weeks)

ii)The biparietal diameter .The maximum diameter between the two sides of the head (13 to 20 weeks).

iii)The femur length

iv)Abdominal circumference.

Determination of gestational age by ultrasound merits another article, because it is a very wide topic.

Multiple pregnancies

Ultrasonography is very useful in determining the number of fetuses, especially if there are more than two fetuses.

Localization of the placenta

It is very useful in placenta praevia(Low lying placenta)- This causes painless bleeding and is a great risk to the fetus.

What is the Routine Ultrasound Scan?

All pregnant should ideally have a scan done around 20 weeks of gestation. This is to exclude multiple pregnancies, gross congenital anomalies,determine placental location and to verify dates.

However there is no hard and fast rule as to the number of scans a woman can have during a pregnancy.

Importance of an ultrasound scan around the 34th week

This scan is also very useful as it will evaluate fetal size, assess growth, confirm presentation and localize the placenta. Since this scan is only a few weeks before delivery, the conformation of the presentation (Head , breech etc).

Importance of an ultrasound scan around the 7th week.

This will help exclude ectopics and molar pregnancy, while diagnosing the pregnancy itself.

References

Essential Antenatal Care- A Guide for the Trainee by Deepal S. Weerasekera MS (O$G), MRCOG (UK), FRCS(Ed), Senior Lecturer in Obstetrics $ Gynaecology, Faculty of Medical Sciences, University of Sri Jayewardenepura, Nugegoda, Sri Lanka.

Dewhurst’s Textbook of Obstetrics and Gynaecology for Postgraduates, sixth edition, Edited by D. Keith Edmonds.

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