Preterm birth was significantly related to the history of stillbirth, blood type, history of premature birth, mother’s diseases, and using specific medicines during pregnancy. Although, advances in pediatric medicine and novel methods of infant care have increased the survival chance of preterm infants, the rate of preterm birth has remained almost unchanged and a serious challenge for health system.
The risk of preterm birth in women with a history of stillbirth was higher in the case group comparing with the control group. Soltani et al. showed that the risk of preterm birth in women with a history of stillbirth was four times higher than women without it . Malacova et al. reported that the risk of preterm birth in women with a history of stillbirth was three to four times higher. They also showed that the risk of preterm birth in these women had increased by 10times . A study by Abu Hamad et al. showed a significant relationship between the history of stillbirth and preterm birth .
The results showed that the risk of premature birth in the mothers with a history of preterm birth was higher in the case group comparing with the control group. A study in Gaza Strip showed that there was a significant relationship between the history of preterm birth and premature birth in the next pregnancy . Abaraya et al. reported that women with a history of preterm birth had a higher risk of preterm birth comparing with mothers without any history of preterm birth and the difference was significant . Results of other studies have shown that the history of preterm birth has a significant relationship with preterm birth [17–20].
Distribution and diversity of ABO blood types depends on ethnicity, race and geographical region. For instant, the most common blood type in European countries and Japan is A and in the USA is O . The findings showed that the participants with AB blood type in the case group had the highest risk of preterm birth comparing with the other blood types. This finding is inconsistent with [21, 22]. The differences in the finding might be due to sample groups, ethical differences, and demographical differences .
The mothers who used specific drugs due to a disease in pregnancy had a higher risk of preterm birth. Li et al. showed that the risk of preterm birth in mothers who used hormonal drugs (OR; 2.23), blood pressure drugs (OR:7.74), and other medications (OR:2.15) during pregnancy was higher than those who did not use any medicine during pregnancy. This finding is consistent with our findings . A meta-analysis study showed that the risk of preterm birth in the mother who used antidepressants (OR:1.16) during pregnancy was higher than other women . A study by Huang et al. showed that mothers who used antidepressant had a higher risk of preterm birth (OR:1.69) .
The risk of preterm birth in the mothers with a history of pregnancy diseases was higher. A metanalysis based on population in 2007 showed that the risk of preterm birth in women with inflammatory bowel disease (IBD) was 1.87times higher than healthy mothers . A study in Asia reported that the risk of preterm birth in mother with IBD was higher . Several studies have shown that mothers with hypertension have a higher risk of preterm birth comparing with healthy women [18, 28–33]. In addition, the risk of preterm birth in mothers with diabetes and those who did not give diabetes tests during pregnancy was higher than the mothers without diabetes [31–34]. Sibai et al. reported that the risk of preterm birth before the 35th week in women with and without diabetes was 9% and 4.5% respectively—i.e. diabetes is a risk factor of preterm birth . A study by Roozbeh et al showed that urinary ducts infection during pregnancy (35.8%) and PROM (30.3%) were among the effective risk factors in preterm birth . The results showed that there was a significant relationship between mother’s disease (e.g. diabetes, hypertension, UTI, PID, and nutritional anemia) and preterm birth [15, 37]. Other studies have shown that the side-effects and diseases like eclampsia and placental abruption are among the risk factors of preterm birth [38, 39]. In general, premature birth is related to the side-effects of pregnancy such as placental bleeding, higher blood pressure in pregnant women, and preeclampsia as these factor limit intrauterus development . In addition, the side-effects of pregnancy hypertension may damage placental veins and stimulate oxytocin receptors, which leads to preterm delivery. Other disease like urinary ducts infections weaken amniotic membrane around the embryo and this leads to PROM and preterm birth . Because these associations were inconclusive, we did not consider these factors in our study. Due to the above mentioned limitations, further studies are required for causality between risk factors and preterm births and increase sample size.