Gestation period. Contrast and compare the nine month gestation period within humans, against other mammals. Why is there a difference in time-period?
Gestation period in mammals is described as the time required for the development of the fetus starting from fertilization and ending at delivery. This period varies in between species. Majority of biological characteristics of animals are linked to their evolutionary survival value. Gestation is dependent on brain size. But the human gestation period is 266 days, 8 days short of 9 months, which is shorter than expected. Therefore, the human brain continues to develop even after birth1.
The gestational period in mammals reveal that the period is strongly correlated to body size. For example, Asian elephants have a gestational period of 645 days and for small animals such as rabbits, the period is about 33 days and for mice about 20 days. So a big animal such as elephant takes a longer time for the fetus to develop than a smaller animal such as mouse1.
However, for bigger animals, the resources are also greater to promote growth of the fetus; still it takes a longer time. If the dimensions of an animal are doubled, it would have eight times the volume and thus eight times the weight of the smaller animal. But the cross sectional area is only four times greater. The blood pressure of the larger animal can be double that of the smaller animal which is again balanced by the length of the umbilical cord which would also be twice and would therefore offer a two-fold greater resistance to flow of nutrients into the developing fetus. So, all other factors being equal, it would take twice as long for nutrients for the eight times larger fetus to pass through an umbilical cord of four times greater capacity. The gestation period, hence, can be stated to be proportional to the scale of an animal and the differences in gestation period amongst different species is attributed to its physical size as well as the scale of the umbilical cord1. Species with a specific breeding season give birth at time when adequate food is available. The gestation period of certain fur bearing carnivorous animals like martens and weasels is further prolonged by a developmental arrest of the embryo delaying implantation in the uterus2.
Various factors govern the subtle differences in gestation period within species. In dogs, the gestation period is positively correlated with litter size. Human pregnancy is divided into first second and third trimesters, each of which lasts approximately for 3 months. After 8 weeks of development, the embryo is called a fetus. The average gestational period in humans is 40 weeks which varies from 37 to 42 weeks3. Human gestation period depends on the sex of the developing fetus. Male fetuses normally gestate three to four days longer than their female counterparts. Multiple pregnancies gestate shorter2. Gestation period is dependent on maternal age as older women have comparatively shorter gestation period4,5. The gestation period in humans also varies among different ethnic groups which, in turn, have evolutionary significance. Asians and Africans have shorter pregnancy duration than the Europeans. The median gestational age at birth is about 39 weeks for the Asians and Africans and is about 40 weeks for white Europeans. This earlier maturation of the fetus in Black races is correlated to the smaller maternal pelvic sizes. The smaller pelvis size aids the female in maintenance of correct posture and stability while running. But, the smaller pelvis has negative impact in terms of causing obstructed labor and maternal mortality. In fact, the necessity of emergency caesarian delivery is quite higher in Africans. Preterm birth is defined to be the birth of an infant at less than 37 weeks of gestational age. The mortality of African fetuses is observed to be lower than the Europeans upto 37 weeks of development but after that, the scenario is reversed requiring preterm delivery supporting the fact that the African fetuses develop faster. A large head is always advantageous for a fetus as it is a symbol of better development of the brain. But it leads to obstruction in delivery especially when the pelvic size is smaller leading to a conflict in maternal-fetal relationship pretty much evident in case of the Africans. Therefore, faster delivery, hence shorter gestation time, is an evolutionarily selected advantage to avoid such complications in delivery6. The length of the gestation period in humans is affected by imposed external conditions as well. A study reveals that women with poor socio-economic conditions and psychological stress necessitate preterm birth. However, duration of gestation is unaffected by addictive habits like smoking, caffeine and alcohol consumption4.