Fetal Assessment in the Third Trimester
As Lockwood and Magriples (2018) point out, the goal of prenatal care is to ensure the birth of a healthy baby with minimal risk for the mother. Prenatal care is not a single intervention but rather a series of assessments, discussions, education and interventions over time (Lockwood & Magriples, 2018). Antepartum testing is based on the premise that the fetus responds to slowly progressive (chronic) hypoxemia with a detectable sequence of biophysical changes that begin with signs of physiological adaptation and potentially ending with signs of physiological decompensation (Signore & Spong, 2018). Fetal biophysical activities, such as heart rate, movement, breathing, and tone, are sensitive to fetal oxygenation and pH levels. Fetal Assessment in the Third Trimester Changes in fetal biophysical activities occur in response to, or in association with, hypoxemia and acidemia (Signore & Spong, 2018). There are other factors that can affect fetal biophysical parameters that include gestational age, maternal medication, maternal smoking, fetal sleep-wake cycles, and fetal disease or anomalies (Signore & Spong, 2018). Fetal Assessment in the Third Trimester
Essential elements of a fetal assessment during the last trimester of pregnancy include measurements and procedures done on the mother. At each prenatal visit measurement of blood pressure should be obtained. Measurement of blood pressure is necessary for diagnosis of hypertension and to help assess development of preeclampsia or gestational hypertension. Preeclampsia affects the arteries carrying blood to the placenta. If the placenta doesn’t get enough blood, the fetus receives inadequate blood and oxygen and fewer nutrients. This can lead to slow growth known as fetal growth restriction, low birth weight or preterm birth. (Signore & Spong, 2018)If left untreated, preeclampsia can lead to serious, sometimes fatal, complications for both mother and fetus. Fetal Assessment in the Third Trimester
During the second and third trimesters, fetal growth is assessed through measurement of fundal height or by ultrasound evaluation for women with risk factors for intrauterine growth restriction (Lockwood & Magriples, 2018). Carroll and White (1988) indicate that this measurement in centimeters should correspond with the number of weeks gestation. The closest correlation occurs between 18 and 30 weeks. Serial measurements should be plotted to allow comparison and to allow for detection of abnormal growth patterns as seen in intrauterine growth restriction (IUGR). Significant deviation from the range of normal requires ultrasound investigation (Carroll & White, 1988).
Fetal heart rate along with assessment of maternal perception of fetal activity should also be assessed at each visit in the third trimester (Lockwood & Magriples, 2018). Signore and Song (2018) state that there is evidence that fetal movement decreases in response to hypoxia. Decreased or absent fetal movements are associated with increased risk of stillbirth or poor neonatal outcome (Carroll & White, 1988). Normal fetuses move for periods of 20-60 minutes every 90 minutes on average. At term, they may not move for up to 75 minutes (Carroll & White, 1988). For women who report decreased fetal movement a nonstress test may be performed (Signore & Spong, 2018). Carroll and White (1998) point out that the NST is only indicative of fetal health at the time of the test. Fetal Assessment in the Third Trimester
Other assessments should include asking about significant event since the last visit, such as recent travel, illness, stressors, or exposure to infection (Zika virus) (Lockwood & Magriples, 2018). Between the 24th and 28th weeks of gestation, women should be screened for gestational diabetes. If the woman is Rh negative, she should receive anti-D immune globulin (Rhogam) (Lockwood & Magriples, 2018). Other screening tests include hemoglobin or hematocrit to check for anemia and screening for sexually transmitted infections and group B beta-hemolytic streptococcus.
Carroll, J. C., & White, D. G. (1988, September). Fetal assessment in the third trimester. Canadian Family Physician, 34, 2005-2009, 2049. Retrieved from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2219138/
Lockwood, C. J., & Magriples, U. (2018, October 25). Prenatal care: Second and third trimesters. UpToDate. Retrieved from https://www.uptodate.com/contents/prenatal-care-second-and-third-trimesters
Signore, C., & Spong, C. (2018, December 3). Overview of antepartum fetal surveillance. UpToDate. Retrieved from https://www.uptodate.com/contents/overview-of-antepartum-fetal-surveillance?search=prenatal%20screening&source=search_result&selectedTitle=4~150&usage_type=default&display_rank=4#H17 Fetal Assessment in the Third Trimester