During in vitro fertilization and embryo transfer (IVF-ET) cycles, clinicians and infertile couples are eager to know pregnancy outcomes as accurately and quickly as possible [1]. In most infertility clinics, the initial establishment of pregnancy after ET is assessed by measurement of the serum level of human chorionic gonadotropin (HCG). A single serum HCG assay after ET is predictive of pregnancy outcomes and assists in the planning of subsequent follow-up [234]. These studies demonstrated that a single HCG measurement after embryo transfer could predict a viable pregnancy with good sensitivity and specificity. However, by simply measuring HCG, it is difficult to distinguish viable pregnancy from biochemical pregnancy, miscarriage, and ectopic pregnancy [5].

Nomograms are considered useful for physicians during their decision-making processes [6]. In the past, only the cut-off value could be presented. On the other hand, using a nomogram, clinicians can determine the numerical value of probability according to each result. In our previous study, we reported for the first time a nomogram for predicting ongoing pregnancy after IVF-ET using patient age and serum HCG and progesterone levels at 14 days after oocyte pick-up (OPU) [7]. We established our nomogram using a modeling dataset that consisted of 103 infertile women who underwent IVF-ET cycles between April 2011 and December 2012. However, the number of women included in our previous study to develop the nomogram was relatively small. Moreover, we also understood that in many infertility centers, the serum level of progesterone at 14 days post-OPU is not routinely measured. Therefore, the nomogram we previously developed was not widely adopted.

Hence, to reinforce our nomogram, we attempted another analysis of more patients while reevaluating the necessity of serum progesterone measurements for predicting ongoing pregnancy. The main aim of this study was to develop a more accurate and convenient nomogram for predicting ongoing pregnancy after IVF-ET using patient age and serum biomarkers.

The basic clinical characteristics of our participants are shown in Table 1. The mean age was 36.3±4.4 years (range, 24–46 years), while the mean number of transferred embryos was 2.0±0.8. Rates of clinical pregnancy per ET and ongoing pregnancy per ET were 29.9% and 23.6%, respectively.

As shown in Table 2, the initial serum HCG titer was elevated in 97 women. Among them, 12 ultimately had a chemical pregnancy, while 18 experienced a clinical abortion. The mean patient age in the ongoing pregnancy group was significantly lower than those of other groups. The serum HCG and progesterone levels in the clinical pregnancy group were significantly higher than those in the chemical pregnancy group.

Patient age, serum HCG level, and serum progesterone level were used as predictive variables, and ongoing pregnancy was used as the dependent variable. The results of a univariate logistic regression demonstrated that all predictive variables were significantly correlated with ongoing pregnancy. The odds ratio (OR) and 95% confidence interval (CI) were 0.86 (0.79–0.92) for age; 1.02 (1.02–1.03) for HCG; and 1.05 (1.03–1.06) for progesterone.

As shown in Table 3, patient age and the serum HCG and progesterone levels at 14 days post-OPU were all good predictors of ongoing pregnancy. The cut-off value and area under the curve (AUC) (95% CI) were 36.5 years and 0.666 (0.599–0.733), respectively, for patient age; 67.8 mIU/mL and 0.969 (0.951–0.987) for serum HCG; and 29.8 ng/mL and 0.883 (0.840–0.925) for serum progesterone. Among the 3 variables, the serum HCG level was the most reliable marker for predicting ongoing pregnancy, a finding that was consistent with our previous study [7].

When the prediction model was constructed using all three variables – patient age, serum HCG and progesterone levels — the AUC was 0.975 for patient age and HCG (R²=0.6076166). Addition of serum progesterone level to the prediction model did not increase the overall predictability. Furthermore, a high linear co-relationship (r=0.65) was found between the serum HCG and progesterone levels (Fig. 1). Therefore, it is reasonable to exclude serum progesterone level to develop a new nomogram.

The newly developed nomogram for predicting ongoing pregnancy after IVF-ET cycles using patient age and serum HCG level is demonstrated in Fig. 2. Each point is first assigned by a vertical extension (to top points bar) of patient age and serum HCG level. The total point is obtained by adding up the points identified on the point scale for each variable. The total points projected on the bottom scale indicates the probability of ongoing pregnancy. The nomogram indicates that the probability of ongoing pregnancy increases with increasing serum HCG level; conversely, older age is associated with a reduced chance of ongoing pregnancy. The equation of the nomogram is as follows.

Table 4 shows the probability (%) of ongoing pregnancy calculated by our newly reinforced nomogram.

In previous studies, HCG level was measured to predict pregnancy outcomes within the range of seven to fourteen days post-embryo transfer [234]. During early pregnancy, the serum HCG and progesterone levels change rapidly and consistently [8]. Therefore, it is essential to take measurements at a fixed point in time. In the present study, serum HCG and progesterone levels were measured at the time of the pregnancy test (exactly 14 days post-OPU).

It is well known that female age is one of the most important independent factors strongly related to implantation rate [9]. However, most studies conducted to date have only analyzed HCG levels without considering age. The present study demonstrated that mean patient age in the ongoing pregnancy group was significantly lower than those in the other groups. The predictive power for ongoing pregnancy was relatively low compared to serum HCG or progesterone levels, but the predictive power was significant (AUC, 0.666; 95% CI, 0.599–0.733). Therefore, patient age should be considered for predicting pregnancy outcomes.

Progesterone plays a crucial role in maintaining pregnancy by its endocrine and immunological effects [10]. Hence, we initially expected that the serum progesterone level would influence the discrimination of clinical abortion in women with intrauterine pregnancy. However, contrary to our initial expectation, serum progesterone level did not differ between the clinical abortion and ongoing pregnancy groups. When constructing the prediction model, adding serum progesterone level did not improve overall predictability. A recent study [3] also showed that the AUC of HCG only for biochemical pregnancy is not increased by the addition of progesterone measurement (0.936 and 0.937, respectively). Furthermore, the linear relationship between HCG and progesterone was high, suggesting that multi-co-linearity is present in the model. Thus, we decided to exclude serum progesterone level in the new model. Hence, the newly developed nomogram presented in this paper used only patient age and serum HCG level and omits serum progesterone level. We believe that this new nomogram offers greater benefits because fewer variables in a nomogram generally indicate greater convenience. Moreover, with respect to cost, the need for fewer laboratory tests decreases the burden for both physicians and patients.

Only one nomogram to predict pregnancy outcomes after IVF-ET has been published by our research team to date [7]. However, the number of women included in the study was relatively small (n=103). The present study was conducted with large data sets (n=284). The newly developed nomogram presented in this paper showed a much higher accuracy for predicting ongoing pregnancy than our previous version of the nomogram, in which a combination of three variables (patient age, serum HCG level, serum progesterone level) showed an AUC of 0.912 (95% CI, 0.815–1.000), sensitivity of 89.3%, specificity of 80.0%, positive predictive value of 89.3%, and negative predictive value of 80.0%. In the present study, a combination of two variables (patient age and serum HCG titer) showed a higher AUC of 0.975 (95% CI, 0.954–0.988), sensitivity of 98.5%, specificity of 90.8%, positive predictive value of 0.5%, negative predictive value of 23.3%, and the highest McFadden R² value (0.6076166). We identified an average AUC increase of 0.004 compared with HCG alone.

In conclusion, we developed a novel nomogram that could be used to estimate the individualized probability of ongoing pregnancy using patient age and serum HCG level measured at 14 days post-OPU. This could better help clinicians and patients forecast ongoing pregnancy after IVF-ET if the serum HCG level is ≥5 IU/L at 14 days post-OPU.