72010 P1130033

1.3.2,5 Twin pregnancy

An important indication for the use of ultrasonogra-phy is the timely diagnosis of a twin pregnancy. The fre-quency with which twin pregnancies are diagnosed de-pends on the time of the examination as well as the breed of the horse involved. It has been reported that the mcidence of twin pregnancies diagnosed between Dąjs 13 and 21 is 15 % in pregnant Thoroughbred mares and only 6 % in pregnant Standardbred mares (Bowman 1986). If the examination is performed later and Warmbloods and draught horses are included, this percentage may drop to 1 to 3 % (Chevalier and Palmer 1982). The detection of twins during the early slage of gęstation reąuires a very carefully executed ultrdsonographic examination of the uterus. The diagnosis is possible with relative ease between Days 12 and 16 (Fig. 1.71).BilateraIly positioned twin conceptuses are not difScult to recognize, provided the examination is carried out conscientiously (Merkt et al. 1983). Difficulties are encountered in cases of ipsilateral twin pregnancies where both conceptuses lie close together m the same hora (Fig. 1.72 to 1.74).

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Many factors can contribute to the faiłure of recog-nizing a twin pregnancy. When two embryonic vesicles lie ciose together the positioning of the scanning probe may make it impossible to see the thin, echoic membranę formed by the two closeiy appositioned sets of veside walk (Fig. 1.73). The presence of twins should suspected when an embryonic vesicle is too large for age as determined by the last service datę of the marę.

If the first examination is pcrformcd after Day 20 the ascent of the embryo with the devclopment of its char-acteristic septum across the middle of the vesicle will have already started. This membranę which is formed jointly by allantoLs and yolk sac can lead to the mis-interpretation of a single ton embryo as a twin (Fig. 1.74). Eąually, a twin with closely appositioned mem-branes of the two vesicles which are misinterpreted as being the allantois yolk sac membranę of a singleton is possible. The hyperechoic linę formed by the apposition of two chorionic membranes usually runs straight and in a vertical direction, whereas the allantois yolk sac membranę commonly lies horizontally (Simpson et al. 1982).

A very significant source of error is the examiner being satisfied with the detection of one embiyonic vesicle and neglects to examine the remaining sections of the uterus for another vesicle. During the phase of embryonic mobility until Day 16 of gestation the embryos freąuently lie in the uterine body where they are easily overlooked. At the time when a singleton embryo is de-tected on Day 14 or 15, another embryo from a second, asynchronous ovulation days later may still be too smali to be found by ultrasonography. Since asynchronous double ovulations with a time interval of 48 to % houts between ovulations can still lead to the conception of a twin pregnancy, the younger of the two embiyonic ves-icles will be 2 to 4 days less advanced than the older and theieby have a diameter smaller than can be detected by ultrasound (Ginther 1986). Endometrial cysts may also lead to a faulty diagnosis of a twin pregnancy (Simpson et al. 1982).