The effect of pregnancy on renal function: physiology and pathophysiology

Urinary iodine concentration during pregnancy in an area of unstable dietary iodine intake in Switzerland

We prospectively investigated urinary iodine concentration (UIC) in pregnant women and in female, non-pregnant controls in the canton of Berne, Switzerland, in 1992. Mean UIC of pregnant women [205 +/- 151 microg iodine/g creatinine (microg l/g Cr); no. = 153] steadily decreased from the first (236 +/- 180 microg l/g Cr; no. = 31) to the third trimester (183 +/- 111 microg l/g Cr, p < 0.0001; no. = 66) and differed significantly from that of the control group (91 +/- 37 microg l/g Cr, p < 0.0001; no. = 119). UIC increased 2.6-fold from levels indicating mild iodine deficiency in controls to the first trimester, demonstrating that high UIC during early gestation does not necessarily reflect a sufficient iodine supply to the overall population. Pregnancy is accompanied by important alterations in the regulation of thyroid function and iodine metabolism. Increased renal iodine clearance during pregnancy may explain increased UIC during early gestation, whereas increased thyroidal iodine clearance as well as the iodine shift from the maternal circulation to the growing fetal-placental unit, which both tend to lower the circulating serum levels of inorganic iodide, probably are the causes of the continuous decrease of UIC over the course of pregnancy. Mean UIC in our control group, as well as in one parallel and several consecutive investigations in the same region in the 1990s, was found to be below the actually recommended threshold, indicating a new tendency towards mild to moderate iodine deficiency. As salt is the main source of dietary iodine in Switzerland, its iodine concentration was therefore increased nationwide in 1998 for the fourth time, following increases in 1922, 1965 and 1980.

Maternal-fetal effects of magnesium sulfate on serum osmolality in pre-eclampsia

OBJECTIVE

To determine the effects of magnesium sulfate therapy on maternal and fetal osmolality in pre-eclampsia.

METHODS

A total of 34 pre-eclamptic women and 22 normal pregnant women participated in the study. Venous blood was drawn upon admission to the labor and delivery unit. Pre-eclamptic patients received standard magnesium sulfate therapy and had a second sample of venous blood drawn 4 h following the beginning of therapy. Fetal umbilical vein blood was obtained immediately following delivery in both groups. Osmolality was measured using a vapor pressure osmometer. Electrolyte levels were measured with a NOVA 8 biomedical analyzer. Data were analyzed via Student's t test, linear regression and correlation with significance established at p < 0.05.

RESULTS

We found no significant difference between the maternal osmolalities of the control and pre-eclamptic groups. Interestingly, fetal cord blood osmolality was significantly lower than maternal osmolality in the normal pregnant women. Sodium levels were also lower, while potassium and ionized calcium levels were higher in the fetal blood. In women with pre-eclampsia treated with magnesium sulfate, there was no difference between the osmolality of the maternal and fetal blood, while potassium and ionized calcium levels were still higher in the fetal blood. Finally, we found no correlation between maternal osmolality and blood pressure.

CONCLUSIONS

High blood pressure in pre-eclampsia is not associated with altered osmolality. An absence of the normal decrease in fetal osmolality is observed in pre-eclamptic women treated with magnesium sulfate.

Urological problems in pregnancy

During pregnancy the urinary tract undergoes extensive anatomical and physiological changes. These changes can result in many symptoms and pathological conditions that may affect the mother and fetus. It is well documented that childbirth may result in urinary tract damage which may predispose to postpartum symptoms. This review describes the physiological and pathological consequences of pregnancy and delivery on the urinary tract, and how these may be minimized.

Renal disease in pregnancy

Women with renal disease who conceive and continue a pregnancy are at significant risk for adverse maternal and fetal outcomes. Risk is inversely related to the degree of renal insufficiency. Pregnancy-induced changes in the urinary tract can temporarily increase renal function compromise, such as nephrosis, but most often results in no net increase in dysfunction. Common complications of pregnancy--such as hypertension and hypovolemia--can be associated with acute renal injury or aggravation of pre-existing disease.

Pregnancy and iodine

Hormonal changes and metabolic demands during pregnancy result in profound alterations in the biochemical parameters of thyroid function. For thyroid economy, the main events occurring during pregnancy are a marked increase in serum thyroxine-binding globulin levels; a marginal decrease in free hormone concentrations (in iodine-sufficient areas) that is significantly amplified when there is iodine restriction or overt iodine deficiency; a frequent trend toward a slight rise in basal thyrotropin (TSH) values between the first trimester and term; a transient stimulation of the maternal thyroid gland by elevated levels of human chorionic gonadotropin (hCG) resulting in a rise in free thyroid hormones and decrement in serum TSH concentrations during the first trimester; and finally, modifications of the peripheral metabolism of maternal thyroid hormones. Together, metabolic changes associated with the progression of gestation in its first half constitute a transient phase from preconception steady state to pregnancy steady state. In order to be met, these metabolic changes require an increased hormonal output by the maternal thyroid gland. Once the new equilibrium is reached, increased hormonal demands are maintained until term, probably through transplacental passage of maternal thyroid hormones and increased turnover of maternal thyroxine (T4), presumably under the influence of the placental (type 3) deiodinase. For healthy pregnant women with iodine sufficiency, the challenge of the maternal thyroid gland is to adjust the hormonal output in order to achieve the new equilibrium state, and thereafter maintain the equilibrium until term. In contrast, the metabolic adjustment cannot easily be reached during pregnancy when the functional capacity of the thyroid gland is impaired because of iodine deficiency. The ideal dietary allowance of iodine recommended by World Health Organization (WHO) is 200 microg of iodine per day for pregnant women. In conditions with iodine restriction, enhanced thyroidal stimulation is revealed by relative hypothyroxinernia and goitrogenesis. Goiters formed during gestation may only partially regress after parturition. Pregnancy, therefore, represents one of the environmental factors that may help explain the higher prevalence of goiter and thyroid disorders in women compared with men. An iodine-deficient status in the mother also leads to goiter formation in the progeny and neuropsycho-intellectual impairment in the offspring. When adequate iodine supplementation is given early during pregnancy, it allows for the correction and almost complete prevention of maternal and neonatal goitrogenesis. In summary, pregnancy is accompanied by profound alterations in the thyroid economy, resulting from a complex combination of factors specific to the pregnant state, which together concur to stimulate the maternal thyroid machinery. Increased thyroidal stimulation induces, in turn, a sequence of events leading from physiological adaptation of the thyroidal economy observed in healthy iodine-sufficient pregnant women to pathological alterations affecting both thyroid function and the anatomical integrity of the thyroid gland, when gestation takes place in conditions with iodine restriction or deficiency: the more severe the iodine deficiency, the more obvious, frequent, and profound the potential maternal and fetal repercussions.

Detection, significance, and therapy of bacteriuria in pregnancy. Update in the managed health care era

Profound physiologic and anatomic changes of the urinary tract during pregnancy contribute to the increased risk for symptomatic urinary tract infection in women with bacteriuria. Asymptomatic bacteriuria is the major risk factor for developing symptomatic UTIs during pregnancy and may be associated with adverse effects on maternal and fetal health. Because most symptomatic UTIs develop in women with bacteriuria earlier in pregnancy, treatment of bacteriuria is undertaken to prevent symptomatic infections. All pregnant women should be screened at the first antenatal visit, which is reliably and inexpensively done with a dipstick culture. Short-course therapy should be given to women with bacteriuria and clearance of bacteriuria should be documented after therapy is complete. Failure to eliminate bacteriuria with repeated therapy or recurrence with the same organism is indicative of renal parenchymal infection or a structural abnormality. All women with persistent bacteriuria or recurrent infection should have follow-up cultures and a urologic evaluation after delivery.

Pregnancy after donor nephrectomy

Potential female donors frequently ask whether unilateral nephrectomy will impair future childbearing capabilities. To address this question, we surveyed 220 women who underwent donor nephrectomy between 1985 and 1992. Of the 144 women who responded, 33 became pregnant after donation for a total of 45 pregnancies. Seventy-five percent of the pregnancies were carried to term without difficulty. Complications incurred during gestation included miscarriage (13.3%), preeclampsia (4.4%), gestational hypertension (4.4%), proteinuria (4.4%), and tubal pregnancy (2.2%). Four of the 45 pregnancies (excluding miscarriages) required preterm hospitalization, resulting in an overall morbidity of 8.8%. There were no pregnancy-related deaths, and no fetal abnormalities were reported. Problems with persistent hypertension, proteinuria, or changes in renal function were not noted. None of the above complications exceeded what has been noted for the general population. Infertility was a problem in 8.3% (3/36) of our respondents, compared with a worldwide incidence of 16.7%. Based on these results, we conclude that donor nephrectomy is not detrimental to the prenatal course or outcome of future pregnancies.

The safety of nitrofurantoin during the first trimester of pregnancy: meta-analysis

Asymptomatic bacteriuria is common during pregnancy and may adversely affect both the mother and her fetus. Nitrofurantoin (NF) has been long recognized as an effective agent in both nonpregnant and pregnant women suffering from urinary tract infections. This meta-analysis was conducted in order to evaluate the safety of NF ingested during early pregnancy. Of twenty-two studies, only four met the inclusion criteria and their analysis could not demonstrate any significant correlation between NF ingestion and fetal malformation. The pooled odds ratio was 1.29 with 95% confidence interval 0.25-6.57. Although the number and quality of the studies included are limited, we thought it important to present the existing data. More extensive controlled studies are urgently needed in order to increase the significance of our study.

Renal electrolyte and water handling in normal pregnancy: possible role of endothelin-1

The study was carried out to determine the urinary excretion of endothelin-1 (ET-1) in normal pregnancy and to define its possible role in mediating the renal response to aldosterone and arginine vasopressin (AVP). Measurements were performed in 12 healthy pregnant women serially in the 20th, 24th, 28th, 32nd and 36th weeks of pregnancy. Urinary ET-1, plasma and urinary aldosterone and AVP levels (RIA methods) as well as plasma and urine sodium, potassium, creatinine and osmolality were measured; creatinine clearance (Ccr), osmolar clearance (Cosm) and free water clearance (CH2O) calculated. Fractional sodium excretion (FENa), urine sodium/potassium ratio (Na/K) and transtubular potassium concentration gradient (TTKG) were also determined. It was demonstrated that urinary ET-1 excretion was higher in pregnant than in non-pregnant women and it increased further as the pregnancy progressed from 34.8 +/- 4.0 pmol/day in week 20 to 44.1 +/- 3.2 pmol/day in week 36 (P < 0.01). Daily ET-1 excretion significantly correlated with AVP (r = 0.39, P < 0.005) and aldosterone excretion (r = 0.62, P < 0.0001). Furthermore, there was a significant positive relationship between ET-1 excretion and urine flow rate (r = 0.67, P < 0.0001), CCR (r = 0.40, P < 0.0025), Cosm (r = 0.58, P < 0.001), sodium (r = 0.56, P < 0.001) and potassium excretion (r = 0.42, P < 0.001). However, such a relationship could not be established between ET-1 excretion and FENa, TTKG and Na/K.(ABSTRACT TRUNCATED AT 250 WORDS)