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Less than 5% of breech babies have their heads in the "star-gazing" position, the face looking straight upwards and the back of the head resting against the back of the neck. Caesarean delivery is absolutely necessary because vaginal birth with the baby's head in this position confers a high risk of spinal cord trauma and death. [28]
In 1944, Reginald Ernest Moreau suggested that in more seasonal environments or higher latitudes, fecundity depends on high mortality. [11] David Lack suggested in 1954 that differential food availability and management across latitudes play a role in offspring and parental fitness. [12]
It is a manual procedure that is recommended by national guidelines for breech presentation of a pregnancy with a single baby, in order to enable vaginal delivery. [2] [3] It is usually performed late in pregnancy, that is, after 36 gestational weeks, [4] preferably 37 weeks, [5] and can even be performed in the early stages of childbirth. [4]
Natural selection is the differential survival and reproduction of individuals due to differences in phenotype.It is a key mechanism of evolution, the change in the heritable traits characteristic of a population over generations.
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The average age of a girl's first period is 12 to 13 (12.5 years in the United States, [6] 12.72 in Canada, [7] 12.9 in the UK [8]) but, in postmenarchal girls, about 80% of the cycles are anovulatory in the first year after menarche, which declines to 50% in the third year, and to 10% by the sixth. [9]
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Survivorship curves can be constructed for a given cohort (a group of individuals of roughly the same age) based on a life table. There are three generalized types of survivorship curves: [ 1 ] Type I or convex curves are characterized by high age-specific survival probability in early and middle life, followed by a rapid decline in survival in ...