gestation

Gestation is the period of development from conception to birth during which an embryo or fetus grows inside the uterus (in placental mammals) or within eggs (in some other species). In humans, gestational age is usually measured from the first day of the last menstrual period (LMP), about two weeks before conception, yielding an approximately 40-week timeline. Some references use conceptional age, which starts at fertilization and is about two weeks shorter.

Gestation consists of several developmental stages: the germinal period (conception to about two weeks) involving fertilization, zygote formation, cleavage, morula, and implantation; the embryonic period (weeks 3–8) with major organ formation; and the fetal period (week 9 to birth) of growth and maturation.

Durations vary by species. In humans term is typically defined as 37–42 weeks of gestation; preterm pregnancies end before 37 weeks, post-term after 42 weeks. In many domestic animals, gestation length is shorter (e.g., dogs 58–68 days, cats 63–65 days, cattle about 280 days).

Key physiological processes include placental development and function, formation of the amniotic sac and fluid, and hormonal changes that sustain pregnancy. Prenatal care, including ultrasound, prenatal testing, nutrition, and avoidance of teratogens, reduces risks. Common complications include miscarriage, ectopic pregnancy, preeclampsia, gestational diabetes, and placenta-related disorders; many pregnancies proceed normally with proper care.