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From Embryology
Maternal pregnancy is divided into three stages of growth and development - the first trimester (foetal weeks 1-12) which involves organogenesis via cellular proliferation, differentiation, and morphogenesis; the second trimester (foetal weeks 13-27) which is centred around organ development and functioning as well as foetal expansion and recognisable movement (quickening); and the third trimester (foetal weeks 28-parturition) during which the foetus undergoes rapid weight gain.