Characteristics of the study population
A hospital-based case-control study was conducted, involving 115 women with preterm birth and 115 controls. Cases and controls were evenly matched by age. Preterm is defined as babies born alive before 37 weeks of pregnancy are completed. Preterm women were divided into three groups based on their gestational age: extremely preterm (less than 28 weeks), very preterm (28 to 31 + 6 weeks) and moderate to late preterm (32 to 36 + 6 weeks). The characteristics of the cases and controls are summarized in Table 1. The majority of the PTD women gave birth moderate to late preterm (80.00%) followed by very preterm (15.65%) and extremely (4.35%). The mean ages of the PTD patients and full term controls were 29.18 (5.14) and 30.14 (4.60) years, respectively (p = 0.14). Cases were more probably to have higher blood pressures, pre-pregnancy BMI and more likely to have caesarian section then controls. All the control patients were multiparous, while this was the first delivery for 38.26% of the PTD women. Interestingly, the women from the study group had a higher BMI at the end of pregnancy (without statistical significance), despite the fact that their pregnancies were on average shorter than in the control group (33.65 (2.74) weeks vs. 38.90 (1.09) in controls, p < 0.001). When analyzing the clinical data of newborns from the studied groups, statistically significant differences were observed between the Apgar scores, newborn weights and placental weights. There were no differences in the frequency distributions of newborns gender between the PTD women and the controls (p = 0.28).
Table 1
Demographic and clinical characteristics of the study population
Characteristics | PTD (n = 115) | Controls (n = 115) | p |
Maternal age, years | 29.18 (5.14) | 30.14 (4.60) | 0.14 |
Gestational age at delivery, weeks — extremely preterm, n, % — very preterm, n, % — moderate to late preterm, n (%) | 33.65 (2.74) 5 (4.35) 18 (15.65) 92 (80.00) | 38.90 (1.09) — — — | < 0.001 |
Systolic blood pressure, mmHg | 110.83 (15.63) | 107.26 (11.28) | 0.049 |
Diastolic blood pressure, mmHg | 70.17 (11.12) | 67.00 (8.35) | 0.02 |
Pre-pregnancy BMI, kg/m2 | 21.43 (2.89) | 20.71 (1.76) | 0.02 |
Post-pregnancy BMI, kg/m2 | 26.15 (3.66) | 25.87 (2.38) | 0.50 |
Caesarean section, n, % | 39 (33.91) | 24 (20.87) | 0.041 |
Primipara, n, % | 44 (38.26) | 0 (0.00) | — |
Birth weight, g | 2375.30 (714.36) | 3473.130 (403.79) | < 0.001 |
Placenta weight, g | 552.01 (149.32) | 626.80 (112.03) | < 0.001 |
Infant male, n, % | 65 (56.52) | 74 (64.35) | 0.281 |
1-min Apgar score | 7.83 (2.68) | 9.86 (0.54) | < 0.001 |
5-min Apgar score | 8.97 (1.44) | 9.98 (0.13) | < 0.001 |
mean ± SD, p - Two Sample t-test, p1 - Pearson's Chi-squared test |
PGR gene polymorphisms and the susceptibility of PTD
As shown in Table 2, the genotype distributions of PGR polymorphisms in the case and control groups were in Hardy-Weinberg equilibrium (p > 0.05). However, there were no differences in allele frequencies between the women with preterm delivery and controls. A frequency of minor alleles in the cases (T2–16.9% and A − 6.1%) were similar to the frequency observed in controls (17.3% and 5.6% respectively).
Table 2
Prevalence of PGR alleles in cases and controls
Variants | Alleles | PTD n = 230 | Controls n = 230 | p |
MAF n (frequency) | HWE p | MAF n (frequency) | HWE p |
ins/del | T1 > T2 | 39 (0.169) | 0.69 | 40 (0.173) | 0.94 | 0.90 |
rs10895068 | G > A | 14 (0.061) | 0.79 | 13 (0.056) | 0.54 | 0.84 |
HWE – Hardy-Weinberg equilibrium χ2 p - Pearson's Chi-squared test |
The genotype distributions of the PGR variants (ins/del and rs10895068) in PTD women and controls are presented in Table 3. There were no differences in frequency of each of the three genotypes, neither for ins/del (T1T1: 68.7% vs. 67.8%, T1T2: 27.8% vs. 30.4%, T2T2: 3.5% vs. 1.7%) nor rs10895068 (GG: 89.6% vs. 87.8%. GA: 9.6 vs. 12.2%, AA: 0.9% vs. 0.0%). Moreover, no significant differences were observed for these two polymorphisms in dominant or recessive models even after adjusting for pre-pregnancy BMI.
Table 3
Genotype distributions of the studied polymorphisms in cases and controls, and their risk prediction for preterm delivery under three genetic models of inheritance
Polymorphism | Controls (n = 115) | PTD (n = 115) | Crude OR ( 95%CI) | p value | AIC | Adjusted OR (95%CI) | p value | AIC |
ins/del |
T1T1 | 79 (68.7) | 78 (67.8) | 1.00 | 0.66 | 324.0 | 1.00 | 0.61 | 320.6 |
T1T2 | 32 (27.8) | 35 (30.4) | 1.11 (0.62–1.96) | 1.06 (0.59–1.90) |
T2T2 | 4 (3.5) | 2 (1.7) | 0.51 (0.09–2.84) | 0.44 (0.08–2.56) |
Dominant | 36 (31.3) | 37 (32.2) | 1.04 (0.60–1.81) | 0.89 | 322.8 | 0.99 (0.56–1.74) | 0.98 | 319.6 |
Recessive | 111 (96.5) | 113 (98.3) | 0.49 (0.09–2.74) | 0.40 | 322.2 | 0.43 (0.07–2.49) | 0.33 | 318.6 |
+ 331G > A (rs10895068) |
GG | 103 (89.6) | 101 (87.8) | 1.00 | 0.67 | 323.1 | 1.00 | 0.43 | 319.9 |
GA | 11 (9.6) | 14 (12.2) | 1.30 (0.56–2.99) | 1.40 (0.60–3.26) |
AA | 1 (0.9) | 0 (0.0) | — | — |
Dominant | 12 (10.4) | 14 (12.2) | 1.19 (0.52–2.70) | 0.68 | 322.7 | 1.30 (0.57–2.98) | 0.53 | 319.2 |
Recessive | 114 (99.1) | 115 (100.0) | — | 1.00 | 321.5 | — | 0.30 | 318.5 |
Abbreviations: AIC Akaike information criteria, adjusted for pre-pregnancy BMI, CI confidence interval, OR odds ratio |
An analysis of the linkage disequilibrium was also performed by using Haploview software. Analyzed variants of the PGR gene (ins/del and rs10895068) are in a linkage equilibrium with D’=1.0 and r2 = 0.01.
Stratified analysis of PGR polymorphisms and the risk of PTD
We carried out stratified analysis to assess the relationship between the PGR variants and the risk of PTD by the subtypes distinguished according to the week of pregnancy termination (Table 4). As the group of women whose pregnancy ended before 28 week (extremely preterm delivery - EPTD) was very small (n = 5), we connected it with the group very preterm delivery (VPTD). Both crude and adjusted subgroup analyses revealed no significant differences for genotype frequencies of studied variants in any genetic models.
Table 4
Observed genotype for progesterone gene variants among women with early (extremely and very) preterm and late preterm births.
| Genotypes/ models | Controls (n = 115) n (%) | E + VPTD (n = 23) n (%) | MPTD (n = 92) n (%) | Controls vs. E + VPTD | Controls vs. MPTD | E + VPTD vs. MPTD |
OR (95%CI) | p | OR (95%CI) | p | OR (95%CI) | p |
ins/del | T1T1 | 79 (68.7) | 16 (69.6) | 62 (67.4) | 1.00 | 1.00 | 1.00 | 0.80 | 1.00 | 1.00 |
T1T2 | 32 (27.8) | 7 (30.4) | 28 (30.4) | 1.08 (0.41–2.87) | | 1.11 (0.61–2.04) | | 1.03 (0.38–2.79) | |
T2T2 | 4 (3.5) | 0 (0.0) | 2 (2.2) | — | | 0.64 (0.11–3.59) | | — | |
| Dominant | 36 (31.3) | 7 (30.4) | 30 (32.6) | 0.96 (0.36–2.54) | 0.93 | 1.06 (0.59–1.91) | 0.84 | 1.11 (0.51–2.97) | 0.84 |
| Recessive | 111 (96.5) | 23 (100.0) | 90 (97.8) | — | 1.00 | 0.62 (0.11–3.44) | 0.57 | — | 1.00 |
+ 331G > A rs10895068 | GG | 103 (89.6) | 21 (91.3) | 80 (87.0) | 1.00 | 1.00 | 1.00 | 0.65 | 1.00 | 0.55 |
GA | 11 (9.6) | 2 (8.7) | 12 (13.0) | 0.89 (0.18–4.32) | | 1.40 (0.59–3.35) | | 1.58 (0.33–7.59) | |
AA | 1 (0.9) | 0 (0.0) | 0 (0.0) | — | | — | | — | |
| Dominant | 12 (10.4) | 2 (8.7) | 12 (13.0) | 0.82 (0.17–3.92) | 0.80 | 1.29 (0.55–3.02) | 0.56 | — | — |
| Recessive | 114 (99.1) | 23 (100.0) | 92 (100.0) | — | 1.00 | — | 1.00 | — | — |
p value - logistic regression adjusted for pre-pregnancy BMI; EPTD – extremely preterm delivery; VPTD – very preterm delivery; MPTD – moderately preterm delivery |
Effect of PGR ins/del and + 331G > A genotypes on week of pregnancy termination
We found no significant difference in week of pregnancy termination before and after adjusted for pre-pregnancy BMI among the genotypes of study polymorphisms in either the patients or the control group (Table 5).
Table 5
Week of pregnancy termination (mean ± SD) according to PGR genotypes in PTD and control subjects.
Variant | Genotypes/ models | PTD | Controls |
mean (SD) | p | P1 | mean ± SD | p | P1 |
ins/del | T1T1 | 33.67 (2.99) | 0.69 | 0.72 | 38.94 (1.08) | 0.78 | 0.77 |
T1T2 | 33.71 (2.18) | 38.78 (1.16) |
T2T2 | 32.00 (0.00) | 39.00 (0.82) |
| Dominant | 33.62 (2.15) | 0.93 | 0.96 | 38.81 (1.12) | 0.55 | 0.53 |
| Recessive | 33.68 (2.76) | 0.39 | 0.42 | 38.89 (1.10) | 0.85 | 0.85 |
+ 331G > A rs10895068 | GG | 33.55 (2.78) | 0.31 | 0.32 | 38.88 (1.09) | 0.60 | 0.57 |
GA | 34.36 (2.41) | 38.91 (1.14) |
AA | — | 40.00 (0.00) |
| Dominant | — | — | — | 39.00 (1.13) | 0.73 | 0.70 |
| Recessive | — | — | — | 38.89 (1.09) | 0.31 | 0.29 |
p1 value adjusted for pre-pregnancy BMI |