lightening

The sensation of decreased abdominal distention during the latter weeks of pregnancy following the descent of the fetal head into the pelvic inlet.

(DOD, NATO) The operation (normally carried out at anchor) of transferring crude oil cargo from a large tanker to a smaller tanker, so reducing the draft of the larger tanker to enable it to enter port.

The noun has 2 meanings:

Meaning #1: descent of the uterus into the pelvic cavity that occurs late in pregnancy; the fetus is said to have dropped

Meaning #2: changing to a lighter color
  Synonym: whitening