In a cycle stimulated as part of infertility therapy, merely measuring the serum hormone levels can neither rule out hy-perstimulated ovaries (Fig. 62.5) nor reliably estimate the number of preovulatory follicles (93). It is for this reason that the number of maturing graafian follicles be monitored sonographically so that therapy can be terminated and contraceptive measures advised when more than two preovulatory follicles develop.
About 5% of women have polycystic ovarian syndrome
(PCOS) caused by inhibited follicular maturation. Its most common cause is adrenal androgen excess. It is characteristic of PCOS for the ovary (91) to contain several small cysts (64), predominantly around the periphery where they form a "pearls on a string" appearance within tissue of increased echogenicity (Fig. 62.6).
An elevated (3-HCG in the maternal serum or urine is an indication of pregnancy, and sonography can confirm the pregnancy. Furthermore, sonography can identify multiple pregnancy (refer to Figs. 66.3, 66.4), which is not always hor-monally recognized, and it can exclude ectopic preçnancv (EP).
Vaginal sonography can detect early intrauterine pregnancy (Fig. 63.1) when the gestational sac (chorionic cavity) measures 2 to 3 mm in diameter. This size is generally found at the beginning of the 4th gestational week plus 3 days after the last menstrual period or 14 days after conception. The initially small cavity grows at a rate of about 1.1 mm per day to become the amniotic cavity (101), in which the embryo (95) is later detectable (Fig. 63.2).
• A gestational sac (chorionic cavity) (101) outside the uterus (39) constitutes an ectopic pregnancy (Fig. 63.3).
Fetal cardiac activity can be detected from the 6th gestational week. At this time, the normal rate is about 80 to 90 beats per minute.
Biophysical limits: According to the guidelines of the American Institute of Ultrasound in Medicine (AIUM), acoustic energies below 100mW/cm: or less than 50 J/cm have no confirmed biologic effect and can be considered safe . Since the sonographic exposure delivered with conventional real time sonography is far below those values, neither thermal nor cavitation effects are to be expected.
The bioeffects are different for
sonography and PW (pulsed-wave) Doppler sonography:
long examination times approach or even exceed the recom long examination times approach or even exceed the recom mended tissue exposure limits. Though no adverse biologic effects have been reported so far with these higher sonographic tissue exposures, it is prudent to refrain from nonessential (color-) Doppler sonography during the sensitive phase of organogenesis (1st trimester) [2, 3].
1 AIUM Bioeffects Committee: Bioeffects considerations for safety in ultrasound. J Ultrasound Med.. Suppl. 7 (198N): 1-38
2 Watchdog-Tutorial: Gepulste Doppler-Geräte—Sicherheitsaspekte. Ultraschall Klin. Prax. 7 (1992): 86-87
3 European Committee for Ultrasound Radiation Safety—the Watchdogs. Transvaginal ultrasonography—safety aspects. Europ J Ultrasound 1 (1994): 355-357
In the sonographic evaluation of pregnancy, biometry is primarily used to asses intrauterine growth retardation but also assists in diagnosing anomalies. The normal biometric values for the gestational age and their percentiles are also found as tables at the end of the book.
Gestational sac (chorionic cavity) diameter (GSD): The initially anechoic chorionic cavity (101) becomes surrounded by an echogenic rim of reactic endometrium (78) (Fig. 64.1) and is detectable after the 14th day of conception. It should be detectable if the serum HCG exceeds about 750-1000 U/l—otherwise an ectopic pregnancy must be excluded (compare p. 63).
The yolk sac is seen as echoic ring-like structure at about the 5th gestational week and increases to 5 mm in size by the 10th gestational week. A diameter of the yolk sac of less than 3 mm or more than 7 mm is associated with a higher risk of developmental anomalies. A yolk sac clearly seen within the uterine cavity excludes an ectopic pregnancy since the yolk
sac is fetal in origin. Figure 64.2 shows a yolk sac (102) adjacent to the spine (35) in a fetus of a gestational age of 7 weeks and 6 days.
Crown-rump length (CRL): A normal fetus is detectable at a gestational age of 6 weeks and 3 days, and has a CRL of approximately 5 mm. At this time, the amniotic cavity measures 15-18 mm. As soon as the fetus (95) is visible, the CRL replaces the GSD since it more accurately determines the gestational age (within the range of a few days) up to the 12th gestational week (Fig. 64.3). Thereafter, measuring the biparietal diameter of the head (BPD) becomes more accurate (compare p. 65).
If a fetus is not detectable in the chorionic cavity as expected for gestational age, the calculation of gestational age should be checked first. If follow-up examination fails to show appropriate development of the still empty chorionic cavity, the finding may indicate a blighted ovum without a developing fetus, which occurs with an incidence of about 5% of all gestations.
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