02 Reproduction in Lower and Higher Animals - part 11 - Pregnancy

02 Reproduction in Lower and Higher Animals - part 11 - Pregnancy


Pregnancy : 
  • It is the condition of carrying one or more embryos in the uterus. It is also called gestation. 
  • It refers to the period between fertilization of the egg, upto parturition. 
  • The average period of pregnancy in human lasts for 266 days from fertilization or 280 days (266+14) counted from LMC- Last Mensturation Cycle. 
  • This pregnancy peroid of approximately nine months is divided into three trimesters of three months each.

First Trimester ( from fertilization to 12th week) :
  • It is the time of most radical changes in mother and embryo. 
  • The embryo receives nutrients in the first 2-4 weeks directly from the endomerium. 
  • It is the main period of organogenesis and the development of body organs. 
  • By the end of eight weeks, the major structures found in the adult are formed in the embryo in a rudimentary form. 
  • The embryo is now called foetus. It is about 3cm long. 
  • Arms, hands, fingers, feet, toes are formed. Foetus can open and close mouth and fists. 
  • CNS is fully formed, working of excretory and cirulatory systems begins. 
  • Movements of foetus begin but mother can not feel it. 
  • Heart beat can be heard from 6th week. Progesterone level becomes high and menstual cycle is suspended till the end of pregnancy. 
  • At the end of first trimester foetus is about 7-10 cm long. 
  • Meanwhile, the mother's body also undergoes rapid changes.
  • High levels of progesterone initiate changes in her reproductive system. 
  • The maternal part of placenta grows, the uterus becomes larger.In this period, the mother experiences `morning sickness' (nausea, vomiting, mood swings, etc).
Second Trimester :  (from 13th to 26th week) :
  • It is the period of rapid growth of foetus. 
  • The uterus grows enough for the pregnancy to become abvious. 
  • The foetus is very active and grows to about 30 cms. 
  • Development of brain begins. Hormone levels stabilize as hCG declines, the corpus luteum deteriorates (regresses) and the placenta completely takes over the production of progesterone which maintains the pregnancy.
  •  Ultrasound (sonography) at 18-20 weeks shows baby's growth and position. 
  • From this estimated due date of delivery can be established. Baby's movements can be easily felt by the mother. 
  • Head has hair, eyebrows and eyelashes appear, pinnae are distinct. 
  • The baby reaches half the size of a newborn. 
Third (final) Trimester : (from 27th week till the parturition) :

  • The foetus grows to about 3-4 kg in weight and 50 cms in length. 
  • Eyes are open. 
  • There is gain in body weight. 
  • As the foetus grows, the uterus expands around it, the mother's abdominal organs become compressed and displaced, leading to frequent urination, digestive blackages and strain in the back muscles. 
  • At the end of third trimester the foetus becomes fully developed and ready for parturition. 

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