Why give a Vit K jab the a baby whose mother had gestational diabetes there to a baby with already thick blood

The short version: the vitamin K shot prevents a rare but devastating bleeding event, and the concern about "thick blood" from gestational diabetes does not change that calculus.

The vitamin K shot exists to prevent vitamin K deficiency bleeding (VKDB), a condition where a newborn's blood cannot clot properly. Without prophylaxis, late VKDB (occurring 2–12 weeks after birth) causes intracranial bleeding in about 7 to 10 per 100,000 live births, and those bleeds often lead to death or severe disability [1]. A single intramuscular dose of vitamin K at birth reduces that risk by more than 99% [1]. The evidence for this is strong: the German surveillance data showed 0.25 cases per 100,000 with IM prophylaxis versus 7.2 without [1].

Your question about "thick blood" points to a real phenomenon. Infants of mothers with gestational diabetes have altered metabolic profiles at birth, including changes in acylcarnitines and amino acids that reflect a different metabolic state [2]. They are also at higher risk for neonatal hypoglycemia, respiratory distress, and NICU admission [3]. But none of the evidence in this retrieval shows that these infants have a clotting disorder that would make them resistant to VKDB or that would make the vitamin K shot dangerous. The concern about "thick blood" is not the same as a documented protection against bleeding.

The vitamin K shot does not "thicken" blood in the sense of causing dangerous clotting. It enables the liver to produce functional clotting factors that are otherwise missing. In healthy adults, even high-dose vitamin K2 supplementation for 30 days did not change any coagulation factor activity or clotting time [4]. The shot restores a normal function the newborn lacks, it does not push the system into a hypercoagulable state.

The real question is whether the risk of VKDB is high enough to justify the injection for every newborn, including those whose mothers had gestational diabetes. The answer from the evidence is yes: the risk of VKDB without prophylaxis is about 1 in 10,000 for late-onset disease, and the consequences are catastrophic. The shot reduces that to near zero. No study in this retrieval shows that infants of diabetic mothers have a different baseline risk or a different response to vitamin K.

My call: the vitamin K shot is warranted for this infant. The concern about thick blood is not supported by evidence of harm from the injection, and the risk of VKDB is real and severe.

Confidence: moderate — the evidence for the shot's effectiveness is strong, but the specific question of infants of diabetic mothers has not been studied directly in the records here.