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.