A system of staging pregnancy that is directly related to normal development is proposed to replace the currently used ‘trimester’ system. If accepted, this concept of staging could be applied across species without the likelihood of a grossly incorrect assessment of the stage of development. In contrast to this, the division of a pregnancy into trimesters is based upon an accident of mathematics and bears no relationship to human development. The proposal is to divide pregnancy into three stages, termed ‘horizons’. First is the ‘embryonic horizon’ from fertilisation until the end of the 8th week from fertilisation, i.e. 10 weeks from the first day of the last menstrual period (LMP). The ‘early fetal horizon’ extends from the end of the embryonic phase to the 25th week of gestation (i.e 25 weeks from the first day of the LMP). This horizon is divided into the ‘primitive phase’ (up to the 15th week of gestation) and the ‘transitional phase’ (from the 15th to the 25th week of gestation). The ‘late fetal horizon’ extends from 25 weeks to beyond term and is divided into the ‘premature phase’ (from 25 weeks to 36 weeks of gestation), the ‘mature phase’ (36 weeks to term) and the ‘post-mature phase’ which extends from term to about 45 weeks of gestation. The importance of this phase is the fact that perinatal morbidity and mortality is proportional to the length of time the gestation continues beyond maturity. It must be stressed that the terms ‘horizon’ and ‘phase’ are intentionally imprecise. This is to emphasise that human development is a continuum and not a series of steps.

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