DEVELOPMENTAL
EVENTS OF THE THIRD TRIMESTER
During the third trimester (27 to 40+ weeks) the fetus continues to grow and develop, and maternal physiology changes in preparation for childbirth. It is most often during this phase of pregnancy that complications such as preeclampsia, bleeding, complications of diabetes or hypertension, abnormalities of growth or amniotic fluid, and preterm labor may emerge.
During the third trimester of
gestation, the dramatic growth of the fetus continues as it attains its final
birth weight and its organs prepare for function as an autonomous individual.
Fetal fat accumulates to provide nutrition and insulation for the first few days
of independent life, accounting for about 15% of fetal weight at term. By the
29th week, the fetus has 300 bones, though eventual fusion of more than 90 of
these fetal growth plates following birth will leave the adult total of 206. At
the beginning of this trimester, in the male, the testes descend into the
scrotum under the guidance of the gubernaculum, which in the female become the
round ligaments supporting the fundus of the uterus. Most babies will settle
into their final birth presentation (cephalic, breech, or transverse) by about
36 weeks’ gestation.
Maternal blood volume increases by
almost twice and cardiac output reaches its maximum. At term, 20% of maternal
cardiac output goes to the uterus and placenta. Late in this trimester, changes
in the cervix prepare for dilation and effacement during labor and delivery. It
is also in the latter portion of this trimester that the number of oxytocin
receptors on the uterine muscle cells increases markedly and there is an
increase in the number of intercellular gap junctions. These micro- pores
between cells provide a mechanism to facilitate the organized and effective
coordinated contractions necessary for successful labor.
Uterine contractions that have been
present since conception become progressively stronger and more noticeable as
the trimester progresses. These are the Braxton-Hicks contractions of late
pregnancy and the contractions of labor and delivery. Amniotic fluid volume
peaks at about a liter at 37 weeks.
As the uterus grows, displacement of
the abdominal contents results in early fullness with meals, heartburn, and
constipation. The growth also results in relocation of the maternal center of
gravity, causing the mother to lean backwards to compensate. This results in
low back pain and the characteristic “duck waddle” of late pregnancy. When the
fetal presenting part begins descent into the maternal pelvis (about 36 weeks’
gestation), causing a decline in fundal height, the patient experiences
improved respiratory and gastric function but at the expense of greater pelvic
pressure and a reduced bladder capacity.
Planning and preparation for
breastfeeding should be undertaken during this trimester. No special physical
preparation is needed for successful breastfeeding, but discussion, questions,
and the acquisition of needed supplies
(e.g., nursing bra) are best taken care of before delivery. Normal amounts of
colostrum have been present from the beginning of this trimester, and some
women experience breast leakage throughout, this period.
For selected patients, “kick counts”
may be used to assess the overall health of the fetus. In general, the
detection of more than four fetal movements over the course of an hour
indicates a healthy fetus. All patients should be
encouraged to monitor their baby’s activity levels and be evaluated for any
prolonged reduction or absence in activity.
Because placental function declines
after 40 weeks, testing of fetal and placental reserve through fetal non-stress
testing, contraction stress testing, biophysical profiles, or measurements of
fetal blood flow in various vessels may be indicated when there are omplications of pregnancy or it extends beyond term.
