Urinary excretion rates of calcium and magnesium in normal and complicated pregnancies

Vitamin D supplementation during pregnancy: safety considerations in the design and interpretation of clinical trials

Maternal-child health benefits of optimizing vitamin D status during pregnancy may include a reduced risk of pre-eclampsia, improved fetal growth and beneficial effects on infant immune function. These hypotheses require evaluation by randomized controlled antenatal vitamin D supplementation trials using doses that are high enough to elevate serum 25-hydroxyvitamin D concentrations into the range believed to be associated with improved health outcomes. Such doses may be considerably higher than the current recommended dietary allowance (600 IU day(-1)) or standard prenatal supplement dose (400 IU day(-1)), and may even be higher than the tolerable upper intake level (4000 IU day(-1)) advised by the Institute of Medicine (2010). A critical review of the published literature yielded limited data regarding the safety of antenatal vitamin D regimens. There have been no published reports of the teratogenic effects of vitamin D on humans. Some animal studies have suggested the potential for dose-dependent fetal toxicities (for example, growth impairment, skeletal malformations and cardiovascular anomalies), but the relevance of these observations to humans is unknown. Antenatal vitamin D supplementation trials should incorporate a range of methods for objectively establishing maternal and fetal safety, and aim to identify the lowest doses of vitamin D required to achieve target outcomes.

Use of 24-hour urinary protein and calcium for prediction of preeclampsia

OBJECTIVE

To assess the efficacy of 24-hour urinary protein and calcium for the prediction of preeclampsia.

MATERIALS AND METHODS

Two hundred normotensive women at 20-28 weeks' gestation were enrolled in the study. All women were asked to collect a 24-hour urine sample. Urinary protein and calcium were measured and expressed as milligrams per 24 hours. Sensitivity, specificity and predictive values were calculated for each test, and cutoff values were calculated using receiver operating characteristic curves.

RESULTS

Twenty-one of the 200 women developed preeclampsia, including eight who developed severe preeclampsia and 13 who developed mild preeclampsia. Compared with the normotensive women (n = 179), the hypertensive patients (n = 21) had significantly lower urinary calcium excretion (167.23 +/- 80.63 mg vs. 277.43 +/- 60.38 mg) and higher proteinuria (351.14 +/- 41.58 mg vs. 296.33 +/- 30.03 mg).

CONCLUSION

A decrease in 24-hour urinary calcium and increase in protein between 20-28 weeks' gestation are risk factors for preeclampsia.

Urinary calcium/creatinine ratio as a predictor of preeclampsia

Hypocalciuria has been associated with preeclampsia (gestational hypertension with proteinuria or other maternal organ dysfunction) but not usually with pure gestational hypertension or normal pregnancy. We hypothesized that hypocalciuria would be a marker of emerging preeclampsia in women presenting with gestational hypertension who later developed preeclampsia. Eighty-one women with de novo hypertension in the second half of pregnancy (n = 81) were enrolled prospectively. At first assessment, calcium/creatinine ratio was determined in a spot urine. Patients were followed until delivery and were classified subsequently according to the occurrence of preeclampsia. Gestational hypertensive patients who became preeclamptic (n = 31) had lower urinary calcium/creatinine ratios at presentation (ratio = 0.07, interquartile range [IQR] = 0.04-0.11) than women who remained as gestational hypertensives (n = 50; ratio = 0.17, IQR = 0.08-0.21; P = .002). Intact plasma parathyroid hormone (PTH) concentrations were similar between groups. Using a receiver operator curve, the best threshold value for the development of preeclampsia was a calcium/creatinine ratio of 0.10, which yielded a sensitivity of only 68% and a specificity of 70%. A low calcium/creatinine ratio preceded the emergence of preeclampsia by 12 (7-24) (median [IQR]) days among a group of women with gestational hypertension. Though this implies primary or secondary disturbances of renal calcium handling even before preeclampsia is clinically apparent, this measurement does not have sufficient sensitivity to recommend its use as a screening test for the emergence of preeclampsia.

Low-sodium diet in pregnancy: effects on blood pressure and maternal nutritional status

In ninety-four Dutch nulliparous women the effects of a low-Na diet in pregnancy on blood pressure, energy and nutrient intake, Ca metabolism, Zn and Mg status and body composition were studied longitudinally. The women were randomly divided into an intervention group (n 41), which used a low-Na diet (mean urinary Na excretion 61 mmol/24 h) from week 14 of pregnancy until delivery and a control group (n 53; mean urinary Na excretion 142 mmol/24 h). No effect of the diet on blood pressure was observed. The use of a low-Na diet resulted in significantly reduced intakes of energy, protein, carbohydrates, fat, Ca, Zn, Mg, Fe and cholesterol. However, the women on the low-Na diet appeared to be able to adapt quite well to the reduced intake since Ca, Zn and Mg homeostasis was maintained. In the case of Ca and Mg this was probably due to the observed reduced urinary excretions of these nutrients. Non-significant reductions in weight gain (1.5 kg) and fat-mass gain (0.9 kg) over pregnancy were found in the women on the low-Na diet. No significant effects of the diet on birth weight or placental weight were observed.

Calcium-phosphorus-magnesium homeostasis in women with threatened preterm delivery

OBJECTIVE

The effect of threatened preterm delivery on calcium, phosphorus, magnesium homeostasis in the third trimester of pregnancy was investigated.

METHODS

Serum concentrations of total and ionized calcium, inorganic phosphorus, magnesium, total protein, albumin, total estrogens and human placental lactogen were determined in women with threatened preterm delivery at 29-36 weeks of gestation (the studied group) and in women with uncomplicated pregnancy of the same duration (the control group). Additionally, activities of total alkaline phosphatase and heat-stable alkaline phosphatase fraction were measured.

RESULTS

Patients of the studied group compared to the control group showed decreased concentration of total calcium (2.17 +/- 0.09 vs. 2.28 +/- 0.13 mmol/l, P < 0.0005), inorganic phosphorus (1.13 /- 0.27 vs. 1.32 +/- 0.23 mmol/l, P < 0.001) and magnesium (0.64 +/- 0.07 vs. 0.70 +/- 0.10 mmol/l, P < 0.003); they also demonstrated decreased activity of total alkaline phosphatase (70.8 +/- 23.2 vs. 81.9 +/- 14.9 IU/l, P < 0.01) and its heat-stable fraction (30.2 +/- 15.6 vs. 59.6 +/- 14.9 IU/l, P < 0.001). In the studied group no difference was found in concentrations of investigated ions and enzymes between women who delivered at term and women who delivered prematurely. Patients with threatened preterm delivery showed serum deficiency of total calcium, phosphorus and magnesium which might be related to premature uterine contractility but does not predict premature labor by week 36 of gestation (66% of patients delivered at term).

CONCLUSION

The deficiency of minerals and lowered activity of total alkaline phosphatase is observed in women with threatened preterm delivery. Laboratory tests of calcium-phosphorus-magnesium homeostatsis have limited predictive value in regard to the term of delivery in women with threatened preterm delivery.

Calcium metabolism, calcium supplementation and hypertensive disorders of pregnancy

In recent years growing attention has been directed towards the possible role of calcium in the development of pregnancy-induced hypertension and preeclampsia. Several studies describe calcium metabolism in normal and hypertensive pregnancy, but so far, they have shown discrepant and inconsistent results. Intracellular free calcium, which plays an important role in vascular smooth muscle contraction, has been claimed as a pathogenic factor in hypertensive disorders of pregnancy. Although there is discordance in the data, a possible role of intracellular calcium in the development of hypertensive disorders of pregnancy cannot be excluded. Observational studies in pregnant women suggest an inverse association between calcium intake and the incidence of hypertensive disorders of pregnancy. Despite large methodological differences, the results from the calcium supplementation trials support this finding. Although it is rather difficult to isolate the effect of calcium intake from the intake of other mineral elements, results from calcium supplementation trials are supportive for calcium being the most important. Proposed mechanisms by which calcium supplementation may lower blood pressure involve changes in parathyroid hormone (PTH) level, the renin-angiotensin system and calcium as a modifier of vascular agent regulation, but none of these have yet been elucidated. At present, circumstantial evidence suggest a positive role for calcium in the prevention of hypertensive disorders of pregnancy, but definite evidence is lacking and further research is warranted.

A prospective study of several potential biologic markers for early prediction of the development of preeclampsia

OBJECTIVE

The purpose of this study was to prospectively evaluate the predictive performance of several potential biologic markers of preeclampsia used alone or in combination.

STUDY DESIGN

A prospective cohort of 1366 nulliparous women was followed up longitudinally on three occasions during pregnancy. The predictive performance of the tests, used either alone or in combination (stepwise multiple logistic regression), was assessed and compared with that of the mean arterial pressure.

RESULTS

Preeclampsia occurred in 109 of the pregnant women. At a specificity of 80% the sensitivity and the positive and negative predictive values for mean arterial pressure (at a threshold of 87 mm Hg) were 46.6%, 23.5%, and 92.0%, respectively, and the corresponding values for a multiple logistic model at 15 to 24 weeks that included some biologic markers, as well as the mean arterial pressure, were 57.1%, 26.9%, and 93.7%, respectively.

CONCLUSION

Preeclampsia can be predicted by a combination of simple biologic tests with a performance similar to second-trimester mean arterial pressure. However, this procedure is insufficient in terms of clinical usefulness.

Urinary calcium/creatinine ratio in the prediction of preeclampsia

The prediction of preeclampsia by the urinary calcium/creatinine ratio during the early third trimester was assessed in 190 primigravidas, aged less than 35 years and between 28 and 32 weeks' gestation without pregnancy complications. Preeclampsia developed in 6.8% of the patients. The mean maternal age, gestational age at entry into the study and at delivery, and the average mean arterial blood pressure at entry into the study did not differ significantly between the 13 patients with subsequent preeclampsia and the 177 normotensive patients. The patients with preeclampsia did not have significantly less excretion of calcium than the normotensives.

Magnesium intake and status and pregnancy outcome in a Danish population

OBJECTIVE

To investigate the relation between pregnancy outcome and magnesium intake and status.

DESIGN

A prospective observational study in which the estimate of nutrient intake and serum samples were obtained before the final outcome of pregnancy was known. A second part of the investigation was a cross-sectional comparison of magnesium status of normal pregnant women and women with certain pathological pregnancies.

SETTING

Antenatal clinic associated to the University Clinic of Obstetrics and Gynaecology, University of Aarhus, Denmark.

SUBJECTS

1203 consecutive women in the 30th week gestation were invited to participate in the study, 991 (82.4%) accepted and records for 965 (80.2%) were available for analysis.

DATA

Food intake data were collected by a combination of a self administered questionnaire and a structured interview of a dietary history type. Serum samples were obtained from blood withdrawn in the 30th and 37th week gestation. Obstetric data were collected from the midwifery and hospital records. For a second part of the investigation, biopsies from the uterus and the abdominal rectus muscle were obtained from women delivered by caesarean section.

RESULTS

In the women with normal pregnancies and in the three groups of pathological pregnancies (pre-eclampsia, SGA-infants and preterm labour) the average magnesium intake was within the range of 200 to 208 mg per 1000 kcal per day (SD 32) and there were no differences between the groups. Neither were differences in serum magnesium observed. Birthweight for gestational age was not influenced by magnesium intake. No differences in magnesium status as assessed by determination of magnesium content in muscle biopsies were found between the normal pregnancies and women with pre-eclampsia, SGA-infants or preterm labour.

CONCLUSION

The intake of magnesium seemed adequate, and pre-eclampsia, SGA-infants, or preterm labour did not seem associated with a low dietary intake of magnesium or magnesium deficiency. Routine magnesium supplementation during pregnancy is not recommended for populations of relative good socio-economic status.

Early detection of preeclampsia

The signs and symptoms of preeclampsia are usually apparent at a relatively late stage in pregnancy (late second to early third trimester). However, the disorder results from abnormal interaction between fetal and maternal tissue much earlier in pregnancy, between 8 and 18 weeks' gestation. During the past two decades numerous clinical, biophysical, and biochemical tests have been proposed for the early detection of preeclampsia. Some of these tests are simple, whereas others are invasive; some have been studied extensively, while others are still under clinical investigation. A review of the literature indicates considerable disagreement regarding the sensitivity and predictive values of the various tests studied. The reported differences in the predictive values of these tests may be attributed to one or more of the following: populations studied, definition and prevalence of the disorder, techniques and methodology used in performing these tests, etc. As a result, there is disagreement regarding the ideal screening test to be used for identifying patients for clinical trials dealing with prevention of the disorder.

Hypocalciuria in hypertensive disorder in pregnancy and how to measure it

We studied the urinary calcium excretion in 41 patients in the third trimester of pregnancy. There was a significant decrease in 24-hour calcium excretion in hypertensive and pre-eclamptic patients. The 24-hour calcium excretion correlated well with the calcium to creatinine ratio of a single voided urine sample. We conclude that measuring urinary calcium excretion may be valuable in the care and the study of pregnant patients with hypertensive disorders, and that the 24-hour urinary calcium excretion can be estimated from a single voided urine sample.

Intra-uterine growth retardation is associated with increased levels of magnesium in amniotic fluid

Magnesium (Mg) and zinc (Zn) levels were measured in amniotic fluid of 30 normal pregnancies between the 16th and the 42nd week of amenorrhea and in 15 cases of intra-uterine growth retardation (IUGR) at 30-38th week of pregnancy. While Zn levels were stable throughout physiological pregnancy, the Mg concentration decreased progressively from 1.09 +/- 0.31 mg/100 ml (mean +/- SD) at the end of the second trimester to 0.75 +/- 0.20 (p less than 0.02) at term. In the case of IUGR, Mg levels (1.22 +/- 0.28) were higher than in controls (0.89 +/- 0.27, p less than 0.01) matched for gestational age. No differences in Zn levels were found between the two groups. Although the mechanisms leading to Mg accumulation in the amniotic fluid of growth-retarded fetuses remain unknown, these data support the role played by Mg in fetal growth.