Ultrasonogram in pregnancy
Ultrasonography uses probe containing multiple acoustic transducer to send pulse of sound wave in to a material. When the sound wave encounter a material of different density part of sound wave reflected back in to the probe and it is detected as an echo. The time it takes for the echo to travel back to the probe is measured and used to calculate the depth of tissue interface causing the echo. Clinician use a hand held probe (transducer) that is placed directly on and move over the body.
Higher frequency sound wave has smaller wavelength and thus are capable of reflecting or scattering from smaller structures. Higher frequency sound waves also have a larger attenuation coefficient and thus are more readily absorbed in a tissue, limiting the depth of penetration of the sound wave in to the body. Frequency used in medical sonography is 1-18 MHz. For deeper penetration in tissue 3-5 MHz frequency wave are used.
Though ultrasonogram has wide diagnostic and therapeutic practice, its common and most widely diagnostic practice is in pregnancy to see the condition of fetus in mother’s womb. The pregnancy period can be divided in to 3 stages.1-13 weeks, 14-26 and 27-40 weeks are called 1st, 2nd and 3rd trimester respectively. Ultrasonogram can provide a lot of valuable information of fetus and mother.
4-5 wks –ultrasonogram (USG) can detect the embryonic tissue thereby confirm the pregnancy. It can estimate the gestational age of the embryo and from this count it can give information about the probable date of delivery. At this stage USG can delineate the presence of twin pregnancy. It can detect whether pregnancy is within uterus (normal pregnancy) or outside (ectopic pregnancy).
6-7 weeks- heartbeat of the fetus can be detected at this stage. So an USG in this stage can ensure the viability of the fetus.
11-14 weeks- a few congenital anomalies like Down syndrome can be detected at this stage. After 14 weeks it is difficult to detect Down syndrome.
So a diagnostic ultrasonogram in 1st trimester is very important to confirm the pregnancy and viability of the fetus as well as to detect few congenital anomalies.
18-22 weeks- congenital anomalies of the fetus can be detected at this stage. It is called anomaly scan to have a USG to detect the congenital anomalies of fetus. Sex differentiation can be possible at this stage. Growth of the fetus is measured. Condition of the placenta and status of amniotic fluid are assessed. Any abnormal position of the placenta and deficiency of amniotic fluid will alert the risk of pregnancy.
After 30 weeks- the growth of the fetus, amount of amniotic fluid and position of placenta and fetus in terms of umbilical cord are assessed which provide valuable information which guide the mode of safe delivery whether normal delivery or elective cesarean section to be adapted.
World Health Organization (WHO) technical report series 875 (1998) supports that ultrasound is harmless. “Diagnostic ultrasound is recognized as safe, effective and highly flexible imaging modality capable of providing clinically relevant information about most part of the body in a rapid cost effective fashion.” As the USG is noninvasive and safe procedure, it should be done as a routine procedure by experts only to have a safe pregnancy and safe delivery.
Dr.Romina Sharmin Shanta
Specialist Radiology & Imaging
United Hospital Ltd.