Is oligohydramnios more common during the summer season?
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- Feldman, I., Friger, M., Wiznitzer, A. et al. Arch Gynecol Obstet (2009) 280: 3. doi:10.1007/s00404-008-0848-4
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The main objective of the present study was to determine whether the summer season is a risk factor for oligohydramnios, by comparing the frequency of oligohydramnios during the summer months versus its frequency during the rest of the year.
A retrospective population-based study including all pregnancies of patients with oligohydramnios that delivered during the years 1988–2007 in a tertiary medical center was performed. All non-idiopathic causes for oligohydramnios were excluded from the analysis. Summer months were defined as May to August. A multiple logistic regression model was performed in order to control for confounders.
During the study period, there were 191,558 deliveries of which 4,335 were diagnosed with oligohydramnios. Of these, 1,553 deliveries occurred during the summer months and 2,782 deliveries occurred during the rest of the year. Higher rates of oligohydramnios were found in the summer months as compared to the rest of the year: 2.5 versus 2.1%, odds ratio (OR) = 1.18, 95% confidence interval (CI) 1.1–1.3; P < 0.001. Using a multiple logistic regression model, controlling for confounding variables such as ethnicity, the summer season was noted as an independent risk factor for oligohydramnios (OR = 1.1, 95% CI 1.02–1.21; P < 0.001). Another independent risk factor for oligohydramnios was Bedouin ethnicity (OR = 1.3, 95% CI 1.2–1.4; P = 0.015).
Oligohydramnios is significantly more common during the summer months versus the rest of the year. Moreover, the summer season is an independent risk factor for oligohydramnios.
Amniotic fluid is the nourishing and protecting liquid contained by the amnion of a pregnant woman. The amnion grows and begins to fill, mainly with water, around 2 weeks after fertilization. Ten weeks later, the liquid contains proteins, carbohydrates, lipids and phospholipids, urea and electrolytes, all of which aid in the growth of the fetus. In the late stages of gestation, much of the amniotic fluid consists of fetal urine. Amniotic fluid also protects the developing baby by cushioning against blows to the mother’s abdomen, allows for easier fetal movement, promotes muscular/skeletal development, and helps to protect the fetus from heat loss .
Normally, amniotic fluid volume increases to about 1 L by 36 weeks of gestation, and decreases thereafter. Diminished fluid volume (i.e. oligohydramnios) is defined by an amniotic fluid index of 5 cm or less. The amniotic fluid index is calculated by adding the vertical depths of the largest pocket in each of four equal uterine quadrants . The etiologies of oligohydramnios are divided into five groups of causes: fetal, placental, maternal, pharmacologic and idiopathic causes (these causes will be addressed as exclusion factors) .
Controversy exists in the literature regarding the association between idiopathic oligohydramnios and hot weather. Interestingly, Lotun et al.  noted substantially higher incidence of oligohydramnios during the Paris heat wave. The present study was designed in order to determine whether the summer season is a risk factor for oligohydramnios, by comparing the frequency of oligohydramnios during the summer months versus its frequency during the rest of the year. The second goal was to compare such differences in two diverse ethnic groups living in the Negev, the southern part of Israel: Jewish and Bedouin women .
Materials and methods
A retrospective population-based study including all pregnancies of patients with oligohydramnios who delivered during the years 1988–2007 at the Soroka University Medical Center was performed . Oligohydramnios was basically defined by amniotic fluid index of 5 cm or less. All non-idiopathic causes for oligohydramnios were excluded from the analysis, which include: (1) Fetal causes: chromosomal abnormalities, congenital anomalies, growth restriction, fetal demise, postterm pregnancy (above 42 weeks gestation) and ruptured membranes. (2) Placental causes: abruption and twin-twin transfusion. (3) Maternal causes: uteroplacental insufficiency, hypertensive disorders, and (4) Pharmacological causes like Prostaglandin synthase inhibitors.
The frequency of oligohydramnios was determined in two ethnic groups: Jewish versus Bedouin. The Bedouins have their roots as nomadic tribes in the desert. Today, half of the Bedouin population of the Negev lives in semi-sedentary settlements and the over half lives in several small towns. National health services in Israel provide the same comprehensive prenatal care for both populations . Harsh living conditions in the hot and arid climate of the desert are experienced more often by the Bedouin population, who tend to live in smaller and more remote settlements, unlike the Jewish population who represents any western society (tends to live in larger cities, with benefits such as air-conditioning, etc.).
Other variables collected were maternal age, parity, gestational age at delivery, gender and birth weight. Being close to the huge deserts of the Sahara and Saudi Arabia, the Negev, which is the southern part of Israel, is a semi-arid area. Its weather conditions are strongly dependent on the dominant direction of air streams: western winds blown from surrounding deserts and are accompanied by sharp variations in meteorological state and transportation of large desert air volumes. Summer months were defined as the period of time from May to August.
Data were collected from the perinatal database that consists of information collected uniformly according to pre-defined criteria immediately after delivery by an obstetrician. Skilled medical secretaries examine the information routinely before entering it into the database. Coding is done after assessing the medical prenatal care records as well as the routine hospital documents.
Statistical analysis was performed using the SPSS-14 package. Statistical significance was ascertained using the Chi-Square test for difference in qualitative variables A multivariable logistic regression model was constructed, in order to define independent risk factors associated with idiopathic oligohydramnios while controlling for confounders. Odds ratio (OR) and 95% confidence interval (CI) were computed. P < 0.05 was considered statistically significant.
During the study period, there were 191,558 deliveries of which 4,335 were diagnosed with oligohydramnios. Of these, 1,553 deliveries occurred during the summer months and 2,782 deliveries occurred during the rest of the year.
Clinical characteristics of the study population
Maternity age (years)
27.8 ± 6.1
Gestational age (weeks)
38.7 ± 3.3
2850 ± 684
50.4% males, 49.6% females
55.7% Jewish, 44.7% Bedouin
Incidence of oligohydramnios during summer months versus rest of the year
95% Confidence interval
Ethnicity (Bedouins vs. Jewish)
Interaction: summer months and ethnicity
A multivariable logistic regression model of factors associated with oligohydramnios
Oligohydramnios summer (%)
Oligohydramnios rest of the year (%)
95% Confidence interval
The major finding of our study was that oligohydramnios is significantly more common during the summer months versus the rest of the year. It seems that an association between oligohydramnios and dehydration might explain in part our significant observations. In dehydration, there is a general water loss that may also influence the amniotic fluid volume. This hypothesis was put to the test in several studies but not in large population-based cohorts. Luton et al.  performed a study on the Paris heat wave and oligohydramnios comparing the incidence of oligohydramnios during August 2003 (i.e. the heat wave month) with August of the previous year. A substantially higher frequency of oligohydramnios was found during the Paris heat wave (17.5 vs. 4.4%, P = 0.03). This prevalence was explained by the higher rates of dehydration during the heat stroke. Nevertheless, the study was based on a relatively small number of cases (131 patients).
Our work differs from other studies done in this area in several aspects. First and foremost, this study is based on a very large population database, which includes thousands of cases of oligohydramnios. Second, this study compares two distinct populations (Jewish and Bedouin), living in the same area, and sharing the same level of healthcare services. The Negev desert is unique in its arid and warm climate, and radical climate changes between the seasons and between day and night.
Bedouin ethnicity was noted as an independent risk factor for oligohydramnios. About 60% of Bedouin marriages are consanguineous, the most common pattern being first cousin marriage. Due to this high prevalence of inbreeding, the Bedouin are at a high risk for congenital malformations and autosomal recessive disorder . However, in our study congenital malformations were excluded, thus we attribute the higher prevalence to other factors such as a different lifestyle and living conditions among the Bedouin. The Bedouin population tends to live in more rural and sometimes semi-sedentary settlements (about 50% live in such settlements), and accordingly are more exposed to the harsh environmental conditions prevalent in the Negev desert area. This may be an especially prominent factor during the summer months, which tend to be extremely hot and dry , and are characterized by radical changes of temperature between daytime and nighttime.
In the Jewish population, although there was a significant difference in the incidence of oligohydramnious during the summer months versus the rest of the year, the trend was less prominent than the one found in Bedouins. This may be due to better living conditions and less exposure to the harsh climate.
In conclusion, although residual confounding cannot be ruled out, in our study population, oligohydramnios is significantly more common during the summer months versus the rest of the year. Moreover, the summer season is an independent risk factor for oligohydramnios. Obviously, clinical practice would not change according to the results of the present study, but the statistically significant differences have physiological significance regarding the etiology of oligohydramnios. Further prospective studies should investigate the exact causes of this higher incidence of oligohydramnios during the summer, before effective interventions to lower incidence of oligohydramnious could be formulated.
Conflict of interest statement