noun
premature baby; preterm infant
Refers to a baby born before 37 weeks of gestation. Used in both medical and general contexts.
未熟児は特別なケアが必要です。
Premature babies need special care.
彼女は未熟児として生まれましたが、今は元気です。
She was born as a premature baby, but she is healthy now.
新生児 means newborn baby (up to about 4 weeks old), regardless of gestational age.
Compound of 未熟 (immature, unripe) and 児 (child, infant). The term is a straightforward medical and general descriptor.