My ultrasound shows my son has an enlarged kidney known as fetal pelviectasis. is this lethal?

Doubt it: The advances in fetal monitoring have given rise to new issues that were never evident before. One is the relatively common finding of dilated renal pelvi which will often go away within a few weeks of birth when the system matures more. Persistence or actual enlargement of kidneys over time can mean a blockage in the system.This will be evident on other studies and an US obtained after birth.

No worries: US of the fetus is very good but it is not always able to detect problems when present and sometimes detects problems that either were never there or resolve shortly after birth. This just requires follow up imaging, not to worry! Enjoy your little baby while he is still part of you, and look forward to meeting him when he is born! Don't let this interfere with your joy, babies are amazing.