One of the primary organs attacked by iron overload is the pancreas. Excessive damage to the pancreas will result in irreversible diabetes (glucose intolerance).
Gestational Diabetes is a form of diabetes that affects women during pregnancy. A study published in February of 2013 (download pdf) showed that GH does not cause gestational diabetes, however having the H63D SNP does increase the severity of it by 700%. In other words having the H63D SNP makes a woman 7 times as likely to need to inject insulin to control their gestational diabetes. Note that this is also the case for H63D carriers as well as homozygotes! Pregnant women with the H63D SNP should carefully monitor their dietary iron intake. This information should be discussed with your OB/GYN. The explanation given in the study is this: "Abnormal iron metabolism may lead to excessive iron accumulation during the pregnancy, because there is no physiological blood loss in this period. Iron overload impedes the insulin extraction in the liver, so it causes the development of peripheral hyperinsulinemia. Iron accumulation in β-cells of the pancreas significantly reduces insulin secretion. Free radicals synthesized in reactions with iron injure cells. Severe damage of renal, vessel, heart muscle tissues can speed up the complication development in people with diabetes, including the GDM."