1 Department of Obstetrics and Gynaecology, Oulu University Hospital, FIN-90220 Oulu and 2 The Infertility Clinic of Oulu, the Family Federation of Finland, Finland
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Abstract |
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Key words: cycle fecundity/endometrial thickness/frozen-thawed embryo transfer/ovulation side/pregnancy rate
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Introduction |
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Later studies concerning assisted reproduction have shown that contralateral ovulation in the previous menstrual cycle results in better oocyte retrieval, fertilization, cleavage and embryo transfer rates in both spontaneous and stimulated cycles (Fukuda et al., 1996, 1998
). It has been suggested that the corpus luteum secretes factors which negatively affect the follicular health of the responsive cohort of follicles of the next menstrual cycle (Fukuda et al., 1996
, 1997
, 1998
).
If right-sided ovulation takes place more often than left-sided, it suggests that ipsilateral ovulation is more frequent on the right side. The objective of our study was to investigate the side of ovulation and its impact on cycle fecundity in frozen/thawed embryo replacement cycles.
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Materials and methods |
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In the frozen/thawed embryo transfer protocol used in our clinic, transvaginal ultrasonographic examination was carried out twice during the cycle, first at cycle days 1012. In these examinations, the side and size of the leading (largest) follicle and endometrial thickness were recorded. Endometrial thickness was measured as the thickest part in longitudinal section, including both endometrial layers. After the first ultrasonographic examination, the women began luteinizing hormone (LH) tests on urine samples every morning to confirm ovulation. Embryo transfer was carried out 48 h after the first positive LH test result. On the day of embryo transfer the second ultrasonographic examination took place. Clinical pregnancy was confirmed by ultrasonography after a positive urinary pregnancy test. No attempt was made to measure interobserver or intra-observer reproducibility in order to determine which side ovulation occurred with transvaginal ultrasonography.
The women were divided into two groups according to the side of ovulation. They were characterized by age, body mass index (BMI), number of embryos transferred and main infertility diagnosis (male factor, tubal factor and endometriosis). The embryos originated either from in-vitro fertilization (IVF) or intracytoplasmic sperm injection (ICSI) cycles. The characteristics of the groups are presented in Table I.
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Results |
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On the day of embryo transfer the endometrium tended to be thicker when the leading follicle was on the left side but the difference was not significant. There was no difference in endometrial thickness as regards the first measurement on cycle days 1012.
The pregnancy rate per embryo transfer was 20.9% (57/273) when the leading follicle had been on the right side and 25.0% (51/204) when on the left side (95% CI for the difference between proportions was 0.118 to 0.0353), i.e. not significantly different.
We re-analysed the data according to the age of the subjects. In the age category of 3037 years, covering 288 cycles (Table III), the incidence of left-sided ovulation was 126, significantly lower than right-sided ovulation (P = 0.034). In this category, the endometrial thickness (±SD) was significantly greater on the day of embryo transfer when there had been left-sided ovulation (P = 0.037). In addition, the pregnancy rate per embryo transfer was significantly higher when ovulation had been on the left side (95% CI for the difference between proportions was 0.01220.199, P = 0.035). In women under 30 years or above 37 years there were no such differences in endometrial thickness or pregnancy rate.
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Discussion |
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The only known difference between the right and left ovary lies in the anatomy of the veins. The left ovarian vein drains to the left renal vein and the right ovarian vein to the inferior vena cava. In the broad ligament there is a pampiniform plexus of veins which drains into both the ovarian and uterine trunks, indicating anastomoses between ovarian and uterine venous drainage.
In males, venous drainage from the testis corresponds to female ovarian drainage. Varicocele is a collection of large veins in the spermatic cord. The predominance of left-sided varicocele (the left side is affected in 70100% and the right side in 09% of cases; Takihara et al., 1991) is considered to be due to the anatomical difference in venous drainage. According to one of the main aetiological hypotheses on the background of varicocele, it is believed that the left renal vein becomes trapped and compressed between the abdominal aorta and the superior mesenteric artery when the person is upright, leading to subsequent dilatation of the veins and even retention of venous blood flow (Takihara et al., 1991).
In women, retrograde phlebography has revealed even reflux and retrograde flow in the ovarian veins, resulting first in congestion of ipsilateral pampiniform plexus and finally also on the contralateral side due to anastomoses between the left and right plexus (Giacchetto et al., 1989, 1990
). This reflux has been found to be predominantly present on the left side (Giacchetto et al., 1989
). The reflux seems to result in pelvic varicosities and congestion, which have been found to be related to chronic pelvic pain in women with no other pathological findings concerning their gynaecological status (Beard et al., 1984
).
According to the theory above, it may be that higher venous pressure in the left ovarian vein directs venous drainage from the left ovary more toward the uterus, enabling better absorption of hormones from the veins to the adjacent arteries supplying the uterus (Einer-Jensen, 1990), finally resulting in higher hormone concentrations in the uterus than does drainage from the right side. Higher hormone concentrations in the uterus might explain the thicker endometrium and better pregnancy rates observed in our study.
The results of previous studies suggest that intra-ovarian factors secreted locally, e.g. by the corpus luteum, affect neighbouring follicular growth negatively during the luteal phase of the menstrual cycle (Fukuda et al., 1997). If the venous flow from the left ovary is slower, it might result in a longer clearance period of these factors and subsequently diminish the possibility of ipsilateral left-sided ovulation in the next menstrual cycle, explaining our finding of lower ovulation frequency on the left side.
The results of this study indicated that right-sided ovulation is more frequent than left-sided. In addition, this study gives support to the hypothesis that the side of ovulation has an impact on implantation of the embryo. The left ovary appears to act more effectively than the right one in women between 30 and 37 years of age, as reflected in endometrial thickness at mid-cycle and the pregnancy rate following frozen/thawed embryo transfer.
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Notes |
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References |
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Submitted on November 26, 1999; accepted on March 6, 2000.