Some studies have reported a significant correlation between endometrial thickness and pregnancy rate [9, 18–20]. However, some do not support this view [1, 13]. Our results agreed with previous studies that reported a correlation between endometrial thickness and clinical pregnancy. This clear relationship provided additional evidence to suggest that endometrial thickness is a useful indicator of endometrial receptivity.
Many studies have found a thin endometrium to be associated with a lower implantation rate, but no absolute cutoff for endometrial thickness exists; good pregnancy rates have been reported in cycles with endometrium <6 mm, and a successful pregnancy has been reported with endometrial thickness of only 4 mm . Noyes N et al.  found that clinical pregnancy rate and live birth rate were significantly lower when endometrial thickness was less than 8 mm than when endometrial thickness was ≥9 mm. In the present study, the thinnest endometrial lining for successful clinical pregnancy was 4.8 mm. The clinical pregnancy (25.5%) and implantation (13%) rate in group 1 was significantly lower than groups 2 and 3. The relatively lower pregnancy rate observed in this group suggests that more attention needs to be given to embryos transferred to such patients.
Why does a thinner endometrium result in implantation failure? Casper RF  speculated that it may be related to oxygen tension. When the thickness measured by ultrasound is < 7 mm, the functional layer is thin or absent, and the implanting embryo would be much closer to the spiral arteries and the higher vascularity and oxygen concentrations of the basal endometrium. The high oxygen concentrations near the basal layer could be detrimental compared with the usual low oxygen tension of the surface endometrium.
Weissman et al.  showed that pregnancy rate was significantly lower above a maximum thickness of 14 mm, and they also suggested a possible increase in spontaneous abortion rates. Rashidi et al.  reported no pregnancies with an endometrial thickness >12 mm (n = 9). However, Richter et al.  and Ai-Ghamdi et al.  demonstrated a significant increase in the pregnancy rates as endometrial thickness increased, which was independent of the number and quality of the embryos transferred. In the present study, implantation and pregnancy rate increased with increasing endometrial thickness. Therefore, our findings support some previous studies in which increased endometrial thickness did not have a detrimental effect on clinical outcome. A case report  has described a successful twin IVF pregnancy in a woman with an endometrial stripe measuring 20 mm. In our study, the maximum endometrial thickness for a successful pregnancy was 19.7 mm.
Ultrasound measurement of endometrial pattern has been suggested to predict pregnancy outcome, but consensus has not been reached regarding the importance of either variable. Some studies [10, 25–27] believed that a trilaminar pattern of the endometrium was correlated with higher implantation and pregnancy rates, while other studies did not find a significant relationship between endometrial pattern and pregnancy rate [11, 18, 28, 29].
Our analysis found that significantly decreased implantation and pregnancy rates were observed in women without a triple-line endometrial pattern on the day of hCG administration. Several studies have suggested that a premature secretory endometrial pattern is introduced by the advanced P rise, and this premature conversion has an adverse effect on pregnancy rates. In our study, higher P levels were found in women with patterns C and B compared to those with pattern A (0.79 ng/mol vs. >0.65 ng/mol vs. >0.58 ng/mol, respectively; P < 0.05). However, another team  found that Progesterone receptor-B has stimulatory effects and an increased PR-B expression induced by ovarian stimulation would lead to the persistence of a proliferative endometrium. The delayed endometrial maturation would thus be desynchronized with the stage of embryo development, leading to decreased implantation rates in ART cycles. The exact mechanism for this is not known, and a rational explanation for this phenomenon awaits further study.
Despite a lower pregnancy rate and implantation rate when a homogeneous, hyperechoic pattern is noted, we disagree with some investigators who recommend embryo cryopreservation and subsequent ET in a frozen cycle. We agree with Friedler  that endometrial pattern offers important predictive information but should not be used as an absolute predictor of conception. Therefore, we believe that such patients should be adequately counseled and given the most adaptive advice.
When assessing the combined effect of endometrial thickness and pattern on clinical outcome, we found that the clinical pregnancy and implantation rates were not significantly different between women with patterns A, B and C in group 1 ( P > 0.05), which may indicate that a thinner endometrium represents poor receptivity of the endometrium regardless of endometrial pattern, while Chen et al.  found that a thinner endometrial thickness with a triple line pattern is associated with a higher clinical pregnancy rate compared to a thinner endometrium with no triple line pattern. There was also no difference between the patterns in group 3, and perhaps adequate endometrial thickness (>14 mm) mitigated the detrimental impact of not having a triple line pattern. There was significant difference in clinical pregnancy and implantation rates between women with the three patterns in group 2. These findings were not in accord with previous studies. Check et al.  found that no pregnancies occurred in patients with homogeneous hyperechoic endometrium, and Chen et al.  found that there were no differences in clinical pregnancy rate between patterns when endometrial thickness was ≥7 mm. Our results suggest that endometrial pattern has an effect on pregnancy rate when women have a moderate endometrial thickness (7–14 mm).
There are several possible explanations for these inconsistencies. Most studies assessed endometrial thickness and pattern on the day of or following hCG administration and on the day of oocyte retrieval, while other studies assessed the endometrium on the day of ET, and even fewer assessed it on both the days of hCG injection and ET. Therefore, the optimal timing of endometrial assessment remains unknown. Previous studies found that assessment on the day of hCG might be more useful as a prognostic test given the earlier timing and the absence of P exposure [32, 33].
In addition, it is necessary to note that the correlation between endometrial thickness and pattern and pregnancy outcome shown in our study does not imply a causal relationship. The relationship may merely result from some other factors that are directly responsible for endometrial receptivity (such as blood flow or some other underlying physiological machinery responsible for cyclic endometrial development). Therefore, although some treatments may significantly improve endometrial thickness, such therapies may not necessarily have any clinical benefit in terms of pregnancy rate.
This study has some limitation, the most important of which is that it is retrospective in nature. However, we believe the results are of interest because similar studies have published with conflicting results. A well-designed and powered randomized clinical trial will be needed to confirm this result.