As per Lu and Nightingale, serum magnesium concentrations associated with maternal toxicity (also neonate depression, hypotonia and low Apgar scores) are:

7.0–10.0 mEq/L – Loss of patellar reflex
10.0-13.0 mEq/L – Respiratory depression
15.0-25.0 mEq/L – Altered atrioventricular conduction and (further) complete heart block
>25.0 mEq/L – Cardiac arrest

Complications
Severe hypermagnesemia (levels greater than 12 mg/dL) can lead to cardiovascular complications (hypotension and arrhythmias) and neurological disorder (confusion and lethargy). Higher values of serum magnesium (exceeding 15 mg/dL) can induce cardiac arrest and coma. Causes
Magnesium status depends on three organs: uptake in the intestine, storage in the bone, and excretion in the kidneys. Hypermagnesemia is therefore often due to problems in these organs, mostly the intestine or kidney. Predisposing conditions
Hemolysis, magnesium concentration in red blood cells is approximately three times greater than in serum, therefore hemolysis can increase plasma magnesium. Hypermagnesemia is expected only in massive hemolysis. Chronic kidney disease, excretion of magnesium becomes impaired when creatinine clearance falls below 30 ml/min. However, hypermagnesemia is not a prominent feature of chronic kidney disease unless magnesium intake is increased. Magnesium toxicity from emergency pre-eclampsia treatment during labor and delivery. Other conditions that can predispose to mild hypermagnesemia are diabetic ketoacidosis, adrenal insufficiency, hypothyroidism, hyperparathyroidism, and lithium intoxication. Metabolism
For a detailed description of magnesium homeostasis and metabolism see hypomagnesemia. Diagnosis
Hypermagnesemia is diagnosed by measuring the concentration of magnesium in the blood. Concentrations of magnesium greater than 1.1 mmol/L are considered diagnostic.
Hypermagnesemia is an electrolyte disorder in which there is a high level of magnesium in the blood. Symptoms include weakness, confusion, decreased breathing rate, and decreased reflexes. Complications may include low blood pressure and cardiac arrest.It is typically caused by kidney failure or is treatment-induced such as from antacids that contain magnesium. Less common causes include tumor lysis syndrome, seizures, and prolonged ischemia. Diagnosis is based on a blood level of magnesium greater than 1.1 mmol/L (2.6 mg/dL). It is severe if levels are greater than 2.9 mmol/L (7 mg/dL). Specific electrocardiogram (ECG) changes may be present.Treatment involves stopping the magnesium a person is getting. Treatment when levels are very high include calcium chloride, intravenous normal saline with furosemide, and hemodialysis. Hypermagnesemia is uncommon. Rates among hospitalized patients in renal failure may be as high as 10%. Signs and symptoms
Symptoms include weakness, confusion, decreased breathing rate, and decreased reflexes. As well as nausea, low blood pressure, low blood calcium, abnormal heart rhythms and asystole, dizziness, and sleepiness. Abnormal heart rhythms and asystole are possible complications of hypermagnesemia related to the heart. Magnesium acts as a physiologic calcium blocker, which results in abnormalities of the electrical conduction system of the heart. Consequences related to serum concentration:

4.0 mEq/L – Decreased reflexes
>5.0 mEq/L – Prolonged atrioventricular conduction
>10.0 mEq/L – Complete heart block
>13.0 mEq/L – Cardiac arrestThe therapeutic range for the prevention of the pre-eclamptic uterine contractions is: 4.0–7.0 mEq/L.