Antepartum care (second trimester): clinical approach

Antepartum care during the second trimester, spanning from 14 to 27 weeks and 6 days of gestation, is essential for managing the period of intense fetal growth and development. This care is pivotal in preventing complications and enhancing pregnancy outcomes.

During this stage, patients should undergo several evaluations including an obstetrical ultrasound, genetic counseling, screening for abdominal wall and open neural tube defects, testing for gestational diabetes, possible repeat antibody screening, and education tailored to this trimester.

The initial approach in a second trimester antepartum visit involves a thorough history and physical examination. The history might uncover common symptoms like nausea, vomiting, heartburn, and round ligament pain. Notably, patients often start to feel fetal movements, known as quickening, around 20 weeks, though it can occur as early as 16 weeks. It is also crucial to revisit the first trimester history to rescreen for depression, anxiety, and intimate partner violence at least once each trimester.

The physical exam should monitor weight changes since both inadequate and excessive weight gain can lead to complications. Blood pressure should be regularly checked; persistent elevations before 20 weeks may suggest chronic hypertension, whereas elevations after this point could indicate gestational hypertension. Every visit should include a fetal heart rate assessment using Doppler, and from 20 weeks onwards, measurement of fundal height to monitor uterine growth, which helps screen for conditions like macrosomia and growth restriction.

With the history and physical assessment complete, it’s essential to conduct an obstetric ultrasound between 18 and 22 weeks of gestation. This ultrasound evaluates the cervical length, placental position, and fetal anatomy. A cervical length under 25 mm may suggest a higher risk of preterm birth, and vaginal progesterone might be considered for those with a shortened cervix and a history of preterm birth in a singleton pregnancy. The placental position, whether anterior or posterior, should also be assessed to check for placenta previa or vasa previa. A thorough examination of fetal anatomy is critical for planning any necessary counseling and delivery at a specialized facility in case of anomalies.

If the due date wasn't confirmed in the first trimester, the second trimester ultrasound might serve this purpose. Should there be signs of early onset growth restriction during the fetal anatomy survey, a follow-up growth ultrasound is advised approximately 4 weeks later. High-risk conditions such as pregestational diabetes or a prior child with a cardiac defect necessitate a referral for a fetal echocardiogram for a detailed cardiac assessment, which offers more comprehensive views than a standard fetal anatomy survey. If not all views are clear or abnormalities are detected during the anatomy ultrasound, a repeat scan may be warranted later in the second trimester.

A clinical note: In the second trimester, the due date is adjusted based on a comparison between the gestational age determined by the last menstrual period and fetal biometry, which includes measurements of the fetal head, abdomen, and extremities. For pregnancies between 14 weeks and 15 weeks 6 days, the due date should be revised if the ultrasound gestational age differs by more than 7 days from the menstrual dates. Between 16 weeks and 21 weeks 6 days, the due date should be updated if the discrepancy exceeds 10 days. From 22 weeks to 27 weeks and 6 days, adjust the due date if the difference is more than 14 days.

Remember that the earliest ultrasound provides the most accurate determination of the due date. Therefore, if an initial ultrasound has already been performed, that date should be used to establish the due date, and subsequent ultrasounds should not be used for redating.

Next, genetic counseling for all patients is essential, discussing both screening and diagnostic options. Many patients may have undergone genetic testing in the first trimester; if so, it need not be repeated. However, if not previously done, this should be addressed in the second trimester. For diagnostic purposes, amniocentesis can be conducted any time after 15 weeks of gestation. Similar to chorionic villus sampling in the first trimester, amniocentesis screens for fetal aneuploidies and single-gene disorders, such as DiGeorge syndrome or achondroplasia. Additionally, noninvasive screening tests focus on detecting aneuploidies.

The quad screen, a blood test performed between 15 and 22 weeks, measures levels of alpha fetoprotein, human chorionic gonadotropin, unconjugated estriol, and inhibin A in maternal blood. Non-invasive prenatal testing (NIPT), which can be done at any point in the pregnancy, examines cell-free fetal DNA in maternal blood to screen for aneuploidies and sex chromosome abnormalities, though it is typically performed in the first trimester.

Regarding screening for abdominal wall and open neural tube defects, alpha fetoprotein (AFP), produced by the fetal liver, is key. An increase in AFP levels in maternal serum, due to conditions like open neural tube defects or gastroschisis, suggests these issues. It's important to note that normal MSAFP values do not rule out closed neural tube defects as the tissue covering prevents AFP leakage. While MSAFP is part of the second trimester quad screen, it is not included in the first trimester screen or NIPT. Therefore, even if patients choose NIPT, chorionic villus sampling, or forego genetic testing, screening for open defects using MSAFP is recommended.

MSAFP is only a screening tool and must be followed up with ultrasound to directly visualize the abdomen, spine, and skull. Due to the high quality of today’s ultrasound images, some patients may opt to skip the MSAFP and rely solely on a detailed examination during the routine fetal anatomic survey.

An essential aspect of second trimester care is screening for gestational diabetes using a one-hour oral glucose tolerance test (OGTT), typically between 24 and 28 weeks. This test does not apply to patients with pregestational diabetes. The test involves consuming a glucose-rich drink, followed by measuring blood glucose levels an hour later. If elevated, a more comprehensive 3-hour OGTT is conducted. This starts with a fasting glucose level, then blood glucose measurements are taken at one, two, and three hours after a 100-gram glucose load. A diagnosis of gestational diabetes is made if two or more of these values are abnormal.

Finally, second trimester antibody screening is crucial for all Rh-negative patients. This screening should occur between 24 and 28 weeks, followed by administration of Rh immune globulin at 26 to 28 weeks to prevent alloimmunization in future pregnancies. It's critical to draw the patient's blood for the screening before administering Rh immune globulin, as it can cause a positive antibody screen for up to 10 to 12 weeks. If the antibody screening is negative, then proceed with the Rh immune globulin administration.

That's a valuable point about Rh Immune globulin! Administering it at 28 weeks to cover the duration until 40 weeks maximizes its protective effect. If it's administered earlier for specific indications, waiting 12 weeks before giving a second dose ensures continuous protection.

In addition to these medical interventions, it's crucial to prepare patients for what to expect during the second trimester. The tetanus, diphtheria, and acellular pertussis (Tdap) vaccine is recommended as early as 27 weeks to pass maternal antibodies to the fetus, offering protection until the baby can be vaccinated post-birth.

Also, educating patients on the signs and symptoms of preterm labor and preeclampsia is essential for early detection and intervention. Inform them about common discomforts such as round ligament and lower back pain, and emphasize the importance of contacting labor and delivery for any concerns like contractions, leaking fluid, vaginal bleeding, or decreased fetal movements. This comprehensive approach ensures that patients are well-informed and vigilant, promoting a safer and healthier pregnancy.

Here's the key aspects of second trimester antepartum care comprehensively:

1. **Timing:** Care is provided from 14 weeks to 27 weeks and 6 days of gestation.

2. **Obstetric Ultrasound:** This is essential for assessing cervical length, placental location, and detailed fetal anatomy between 18 and 22 weeks.

3. **Genetic Counseling:** Offered if not previously completed in the first trimester, covering options for both screening and diagnostic testing.

4. **Screening for Defects:** Screening for open neural tube and abdominal wall defects should be conducted, typically using the quad screen and potentially additional ultrasound imaging.

5. **Gestational Diabetes Screening:** Typically performed between 24 and 28 weeks unless there is a pre-existing condition of diabetes.

6. **Rh-negative Patients:** An antibody screen is necessary, followed by administration of Rh immunoglobulin between 26 and 28 weeks to prevent alloimmunization.

7. **Patient Education:** Important to educate about the Tdap vaccination recommended as early as 27 weeks, and to review the warning signs and symptoms of preterm labor and preeclampsia, along with general expectations for this pregnancy stage.

Ref:

"Guidelines for perinatal care, 8th ed" American College of Obstetricians and Gynecologists (2017)

"Committee Opinion No. 700: Methods for estimating the due date" Obstet Gynecol (2017)

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