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)
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
4) external rotation
Fourth Stage - expulsion of placenta.
3. Initiation of parturition
corpus luteum feto-placental
Involvement of fetal pituitary and adrenals
Progesterone block theory