月齢

1. Origins, structure, and basic meaning

• Structure: 月齢(げつれい) means a baby's age in months or the phase of the moon.
• Basic usage: It is used when checking a baby's growth by months of age or checking the moon's phase for observation. Typical patterns are 赤ちゃんの月齢 and 月齢を調べる.

2. Common pitfalls and similar expressions

• Separate the two senses by subject
  • With a baby it means age in months; in lunar observation it means the moon's phase.
• Difference from 年齢
  • 年齢(ねんれい) means age in general, while 月齢 for a person specifically expresses an infant's age in months.