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1.3.3.2 Accessibility of eąuine fetuses by transrectal sonography

During one study involving 162 Thoroughbred and Trotter mares which were 3 to 11 months pregnant it j was possible to reach the fetus by transrectal sono-1 graphy at all stages of gestation (Kahn and Leidl 1987 a). For these examinations a 5 MHz linear ultrasound i probe with a penetration depth of 10 cm was used. The technicai difficulties associated with the ultrasonic ex-amination of eąuine fetuses very much depend on the stage of gestation. In about one half of the mares ex-amined between Days 80 and 100 of gestation the fetuses lay outside the rangę of the ultrasound waves (Fig.

■ 1.90). At this stage the relatively smali fetus had often descended cranioventrally into the uterus bulging far beyond the pełwic brim. In this position it was too far away to be detectable by the ultrasound waves. From the 4th month it became increasingly easier to reach some parts of the fetus so that only 10 to 20 % of all fetuses between the 5th and llth months of gestation remained beyond reach.

The availability of individual fetal body parts for ultrasonic examination also depends heavily on the stage of gestation. The head will move closer to the matemal pehis as pregnancy progresses and was visible in 74 to 83 % of all 6 to 11 month old fetuses in the above men-tioned study (Fig. 1.90). Ln contrast, the thorax, abdo-men and pehós generally move ever further away. The chest and abdominal regions were accessible in 25 to 50 % of fetuses during the 3rd, 4th and 5th months and only in isolated cases thereafter.

The above mentioned results reflect tendencies and can be improved significantly through the development of better ultrasound technology. With improved tech-nology eąuine fetuses and their body parts will then become even morę avaiiable for sonographic examin-ations. It has also been demonstrated that the short-comings of transrectal sonography could be com-pensated for in many cases by the transcutaneous appli-

cation of the ultrasound probe (0’Grady et a). 1981, Pipurs and Adams-Brendemuehl 1984). By applying the probe in the area between the matemal uddcr and navel fetal structures in the proximity of the abdominal wali can be visualized from the outside during the 2nd and 3rd trimesters of gestation. Using this techniąuc the trunk of the fetus remains accessible until birth.

1.3.3.3 Sonographic fetometry in horses

Through fetometry, ultrasonography offers the pos-sibility of determining fetal age and to assess the devel-opment of the fetus and pregnancy (KAhn and Leidl 1987 a). The best approach for measuring a fetus depends on the approximately suspected stage of pregnancy. In principal it is expected that the combination of measurements from as many parameters as possible will result in the most accurate estimation. Until Month 6 the diameter of the trunk and the size of any additional-ly accessible organs should be determined. From the 6th month especially the head should be fetometrically eval-uated. On the procedures for fetometiy see also Chap-ter 2.4.4.

1.3.3.3.1 Eye and braincase

The eye is the fetal organ which is most freąuently available for fetometrical assessment during the entire course of gestation (Fig. 1.90 and 1.92). The largest diameter of the eye increases approximately in a linear fashion as pregnancy progresses (Fig. 1.91).

The expansion of the inner diameter of the cranial cavity increases veiy rapidly (Fig. 1.91 and 1.92) and is well correlated with the stage of pregnancy. The braincase can only be measured until the 8th month of gestation, because thereafter the sound waves are too at-tenuated by the cranial bones and the imaging field of conventional scanners is too smali to allow the cranium to be depicted in toto.