Gestation and Parturition


I.     Gestation refers to the period of development from  fertilization until birth. Mean duration in humans is 40
        weeks (280 days) from the last menstrual period (LMP), or 38 weeks (266 days) from conception.

         Range of gestation in humans 245-294 days (35-42 weeks).

         Premature by date - delivery prior to the 37th week.

         Postmature - 2 or more weeks post EDC.

 Variation in length of gestation can occur due to:

 1. disease, eg. diabetes

 2. multiple pregnancy

 3. fetal defects

 4. nutritional status
 
 

II.  Embryonic and Fetal Development

 1. Embryonic period : zygote - 9 weeks (12 wks)

  Myofibrils of heart mesenchyme begin to contract by 4 wks   p.c. Formative body plan established by 6 wks p.c. Except
  for external genitalia, all external and internal   structures are established. By the end of the 12th week all systems are
  formed;  fetal growth begins.

 2. Fetal growth:

  Conforms to cubic law as related to time: W=a(t-t')3,where W is the weight of fetus on any given day; a is
  a constant expressing rate of nutrient supply/unit of fetal surface area; t = day of gestation and t' is the
  lag period. In humans, a= 0.24x10-6 and t'= 36 (as calculated from 1st day of LMP). When a decreases fetal
  growth retardation occurs.

  Rate of growth is enormous at first, the zygote grows over 106X during the first month; in the last month of gestation the
  rate of increase is only 0.3.

  Decreased growth rate at 38-40 wks and continuing post term may reflect changes in the placenta or failure of
  utero-placental circulation to match total nutritional requirements.
 

III. Factors which affect fetal growth rate

  1. Maternal Nutrition - effects of undernutrition are only important during the last 1/6 of gestation. Maternal overnutrition
      can result in 10% increase in fetal birth weight. Thus, over- or undernutrition affects fetal growth only to a slight
      degree, and only during the last 6-8 wks of pregnancy. Malnutrition (poor quality of diet) increases frequency of
      prematurity, placental abruption, anemia, and toxemia.

  2. Placental insufficiency - pathological changes in placenta which result in IUGR. Can include: gross infarction, avascularity
      of placental villi, fibrin deposition in IVS, etc. Net effect is to reduce the total surface area for exchange.

  3. Impaired utero-placental blood flow - reduction in uterine blood flow causes fetal "starvation".

  4. Endocrine factors - most maternal and placental growth hormones (GH, hPL, PRL, hCG, etc) have little effect
     in altering fetal growth parameters. Fetal hormones also do not seem involved. However,  insulin by regulating glucose
    levels does: diabetic mothers tend to have babies with increased birth weight.

IV. Disorders of embryonic and fetal development

 Normally result in a spontaneous abortion. Caused by multiple  factors which are poorly inderstood; these can include genetic
 and environmental factors.
 

V. Parturition (giving birth)

 1. Terms:

  Gravida - # of pregnancies
  Para    - # of births
  Aborta  - # of abortions (spontaneous or induced)

  Cervical effacement - 0-100%; dialation - FT-10cm
  Station: floating, -3 to +3

  Fetal lie - relationship between long axis of fetus and that of the mother, ie. transverse, longitudinal, oblique.
 
  Presentation - presenting part which may be palpated, eg. brow, face, shoulder, breech

  Position - relationship between presenting part and the maternal pelvis, ie. LOA

  Dystocia - difficult labor.
 

 2. Stages of parturition:

  First Stage - relaxation of pubic symphysis, cervix, pelvic tissues and ligaments. Occurs slightly before or during early
    labor.Controlled by estrogen and relaxin (some involvement by PGF2a).
  Second Stage - beginning of true labor (cervical effacement and dilatation). Uterine contractions cause thinning of lower
    uterine segment and cervix (some contribution by fetal presenting part and membranes).
  Third Stage -  expulsion of fetus. Increased uterine contractions and dilated cervix. Time required depends on parity of
    mother. Increased release of oxytocin.

     Fetus undergoes: 1) flexion
          2) internal rotation
          3) extension
          4) external rotation

  Fourth Stage - expulsion of placenta.

 
 3. Initiation of parturition

                      corpus luteum                                feto-placental

                            goat                                               man

                            rabbit                                           primates

                            mouse                                           sheep

  Involvement of fetal pituitary and adrenals

  Progesterone block theory

  Prostaglandins