Chapter 1. Overview: Second/Third Trimester

Welcome to the section on the overview of the second trimester ultrasound. Performing an ultrasound in the first trimester allows for assessment of pregnancy viability and measurement to determine gestational age and estimate the due date.

Nevertheless, there are numerous situations in which women are unable to obtain care before the second trimester. In such cases, it is essential to have a comprehensive evaluation of the pregnancy, assessing not only gestational age but also additional indicators of maternal and fetal health.

This section offers a brief overview of the mid-Trimester Ultrasound relevant to point-of-care. It does not cover detailed fetal anatomic surveys but demonstrates typically observed features.

OBJECTIVES

By the completion of this Section the participant will be able to:

Describe typical reasons for a second trimester ultrasound in a triage situation for obstetric complications
List the components of a second trimester general evaluation
Accurately describe placental locations on images
Describe briefly the components of obtaining fetal biometry (to be covered in a separate course on Biometry and Growth)
Recognize the components of a fetal anatomic survey, while understanding that this is not typically within the scope of practice for a point-of-care ultrasound

A point-of-care ultrasound (POCUS) performed in the second trimester can provide a great deal of information. A 20-week fetal anatomic survey is typically conducted at this stage, which can detect most abnormalities. If this survey is not accessible, or if a woman has not received prior prenatal care or ultrasound, it may be necessary to assess fetal growth. Details on fetal growth assessment are discussed in a separate course called Biometry and Growth.

Figure 1. Point-of-care indications in second/third trimester.

Non-routine second trimester ultrasound is typically used to assess vaginal bleeding (often for placental location or cervical competency), determine fetal number and presentation, and evaluate fetal growth through biometry when necessary.

Any of these are reasons to consult or collaborate with an advanced practitioner, making the second or third trimester a valuable triage tool.

PLEASE NOTE THAT POCUS EVALUATION DOES NOT INCLUDE A COMPLETE FETAL ANATOMIC SURVEY

Figure 2. Additional indications for POCUS in second/third trimester.

Additional reasons for POCUS during the second and third trimester include assessing fetal growth, well-being, and identification of gross abnormality. since some of these may be missed during an earlier scan.

Second trimester scans also help assess patients with complications for triage in hospitals or clinic settings. Some indications for this triage ultrasound include decreased fetal movement, lagging fundal growth, vaginal bleeding, and signs of preterm labor in women.

One important purpose is to rule out pre-term labor. Typically, assessment of preterm labor involves monitoring for contractions; however, it is also necessary to evaluate for cervical shortening or dilation. Cervical measurement will be addressed in more detail in a subsequent chapter in this course.

As stated previously, fetal anatomic survey is NOT part of a point-of-care ultrasound assessment under current national and international guidelines. Only certain parts of fetal anatomy are described in this course. Only those necessary to find correct imaging planes for measurements are presented, and are covered in the course on Biometry and Growth in this series.

When performing a second trimester scan, these are areas that a provider should evaluate:

Fetal number, viability, position

Cervix

Placenta and cord locations

Amniotic fluid – general assessment

Other questions to be answered:

--Does the baby have adequate movement and heartrate?

--Are there any grossly obvious anatomic variations (fetal or maternal) that require consultation or referral?

--Are the baby’s growth measurements appropriate?

Figure 3. Components of second/third trimester POCUS.

All of the following are recognized components for a point-of-care ultrasound in second/third trimester: ^1,2,3

Fetal number, viability, and position

Cardiac activity

Placentation –location and proximity to cervix

Amniotic fluid volume

Biometry

Anatomic structures noted (structures necessary for performing biometry)

Each of these will be discussed in more detail in subsequent modules.

XX Basic Fetal biometry

Basic fetal biometry is INCLUDED in point of care practice and includes:¹

BPD – Biparietal Diameter

HC – Head Circumference

AC - Abdominal Circumference

FL - Femur Length

EFW - Estimated fetal weight (Calculated from the above measurements)

*NOTE: Detailed information on Biometry is covered in a separate course entitled “Biometry and Fetal Growth”

PERFORMING THE EXAM

Two main areas are studied: the FETUS, and the MATERNAL/FETAL environment, which includes the uterus, fluid, cervix, placenta, and cord.

Figure 4. Performing the POCUS scan in second/third trimester.

Figure 5. How to perform the second/third trimester POCUS.

For a second trimester ultrasound, use a transabdominal transducer (with lower frequency) to penetrate deeper into the abdomen and pelvis.

A curvilinear transducer is the appropriate first choice of transabdominal transducer.

When trying to view the cervix or other deep pelvic structures, the transvaginal transducer will often be necessary. The use of the transvaginal probe is described in the previous courses.

Image 1. Transabdominal scan in second/third trimester

Image 1. shows a transabdominal scan of the pelvis with a full bladder. Anatomical structures are easily seen in this woman who is thin. As the fetus develops, visibility improves.

Considerations

Figure 6. Considerations for performing a second/third trimester POCUS.

During POCUS exam in second or third trimester, it is generally advisable for the bladder to be partially empty. This can reduce discomfort during scanning and helps ensure that pelvic structures are not distorted or displaced by a full bladder. The bladder also serves as an acoustic window to enhance visualizing pelvic structures.

Start by scanning the whole uterus in both sagittal (longitudinal) and transverse planes to identify all fetal and maternal structures, as well as fetal position.

Establish fetal viability first

Image 2. Establish fetal viability first during a POCUS ob exam

Video clip 1. Fetal heart beat.

Assessing fetal viability is an initial step. The video clip above shows fetal cardiac activity. When starting a scan, confirm either the presence of fetal movement or cardiac motion before moving on to examine other structures. Capturing an M-mode image is recommended to document heart rate.

EVALUATING the CERVIX

Figure 7, Image 3. Normal cervical length.

Ensure that the woman’s bladder is not full when assessing the cervix, as this may affect cervical length. A full bladder can falsely stretch and elongate a cervix, making it appear longer than it is. A transvaginal transducer provides the most accurate measurement, but a general assessment can be done without one. This is important to know for those without access to transvaginal imaging.

Pitfalls of performing transabdominal ultrasound

Cervical length - transabdominal

Figure 8., Images 4,5. Cervical length using transabdominal ultrasound.

Figure 8 (above) demonstrates that a very full bladder can distort and elongate the cervix, leading to inaccurate measurements (see left image). A partially full bladder (right image) provides a more accurate cervical measurement.

Cervical length transvaginal

Figure 8., Images 4,5. Cervical length using transabdominal ultasound.

A transvaginal ultrasound is more accurate. If this is unavailable and a short cervix appears on transabdominal scan, refer the patient for a diagnostic ultrasound. Normal cervical length in pregnancy is 3–3.5 cm. Lengths under 2–2.5 cm are considered shortened and require advanced assessment. Individuals with this finding should be referred for further advanced evaluation.^4,5

Cervical length variations by gestational age will be covered in detail in the chapter on Cervix in this course.

Funneling refers to the V-shaped opening of the internal os that occurs when pressure is exerted on this area of the cervix, typically from uterine contraction. This finding is particularly concerning in the presence of increased uterine contraction frequency and intensity, a history of preterm labor or birth, or when the functional (intact) cervical length is reduced to less than 2.5 cm. Cervical measurements and the clinical implications of are addressed later in this course.

Cervical funneling

Images 6, 7. Normal cervix versus funneling cervix.

Placenta locations

Following the first trimester, the general location of the placenta is an important assessment, as well as measuring the proximity to the internal os.¹ An anterior placenta is situated along the anterior wall of the uterus (appearing at the top of the ultrasound screen). A fundal placenta typically appears at the uterine fundus, as visualized in a sagittal plane. Conversely, a posterior placenta is positioned on the posterior uterine wall, adjacent to the maternal spine.

Images 8,9,10. Placenta locations: anterior, fundal, posterior.

Examination of the entire placenta includes assessing the lower edge and its distance from the internal cervical os. A placental edge located less than 2 cm from the internal os is defined as low-lying. Further discussion of low-lying placenta and placenta previa is provided in the chapter on placenta, later in this course.

Figure 9, Image 11. Low-lying placenta.

Placental vascularity and cord insertion

Figure 10, Images 12, 13. Images of placental cord insertion.

If color doppler is available, identifying where the cord inserts into the placenta can be helpful. If it inserts centrally, that is normal. If it inserts toward the outer margin of the placenta, this could have implications for fetal growth later in the pregnancy.

Placenta bleeding/abruption

When a woman presents with bleeding, two primary scenarios can be evaluated via ultrasound. If the placenta detaches or separates from the uterine wall, there may be either active bleeding or no observable hemorrhage. In cases where abdominal pain and uterine rigidity are present, placental abruption should be suspected. This condition constitutes a medical emergency. While some patients may experience a “chronic”, or a “small” abruption and continue to full term, a complete placental abruption requires immediate intervention.

Figure 11. Image 14. Image of placental abruption.

If a total abruption is suspected, immediate assistance should be sought without delay for an ultrasound. Detecting evidence of an abruption can be challenging, even for experienced sonographers, so urgent management should be based on clinical findings instead.

Placental grading

Placental scans check for premature aging, mainly by detecting early calcifications and indentations. Past grading of placentas is now considered clinically relevant only if these signs appear before the third trimester, which may occur in cases of uncontrolled gestational diabetes and fetal macrosomia. Placental grading is covered in detail in the chapter on placenta in this course.

Umbilical Cord

The umbilical cord can be examined in both longitudinal and transverse (cross-sectional) views. In the longitudinal view, twisting of the cord may be observed, which is commonly associated with fetal movement, an assurance of overall fetal well-being. This is generally considered a typical finding. In the cross-sectional view, one larger vessel (the umbilical vein) and two smaller vessels (the umbilical arteries) are usually present. If only two vessels are identified, this can sometimes be linked to certain fetal complications, so further consultation with a specialized provider is recommended if this observation is made.

Figure 12., Images 15, 16. Images of umbilical cord: cross-section and cord twisting.

Amniotic Fluid

Amniotic fluid levels can be generally assessed by how clearly fetal structures appear during scanning. Low fluid results in cloudier, lower-resolution images, as adequate fluid level improve image clarity.

Figure 13, Images 17, 18. Images of low amniotic fluid making imaging more difficult.

Alternatively, when all fetal images are distinctly visualized and the fetus appears to be surrounded by an excessive amount of amniotic fluid, this may suggest polyhydramnios. Assessment of amniotic fluid volume will be addressed in the chapter on Biophysical Profile in this course. It is most predictive if evaluated after 32 weeks’ gestation but can be performed as early as 28-29 weeks’ gestation.

Figure 14., Image 19. Image demonstrating improved resolution with adequate amniotic fluid.

Excessively high fluid levels- polyhydramnios

Polyhydramnios in the second trimester often signals a fetal abnormality. Consultation or referral for advanced assessment is recommended. Excess fluid may suggest a chromosomal disorder, syndrome, or a fetal swallowing issue such as esophageal atresia.

Image 20. Image of large vertical pocket of amniotic fluid, suggestive of polyhydramnios.

Fetal biometry

Table 1 lists the key steps for assessing a baby's gestational age and growth using biometric methods. First trimester ultrasounds are most reliable for due dates, but second trimester scans are suitable if early data is unavailable. This chart provides an overview; each component will be covered in a separate course.

Table 1. Components of basic fetal biometry.

Fetal head

Two measurements of the head involve measuring the distance between each biparietal bone, as well as the overall head circumference.

Images 21, 22. Images demonstrating ultrasound measurement of biparietal diameter and head circumference.

Fetal abdominal circumference

The abdominal circumference must be measured when the fetal abdominal structures of the stomach, spine, and portal vein are all aligned, as shown in the images below (Images 23,24).

Images 23, 24. Images demonstrating ultrasound measurement of abdominal circumference.

Femur Length

Femur length is measured as shown in the images below (images 25,26) .

Images 25, 26. Images demonstrating ultrasound measurement of humerus (left image) and femur (right image).

Some ultrasound reports on fetal biometry may include measurement of the humerus in the arm. This is more commonly done to rule out skeletal abnormalities rather than contributing to growth calculations. However, measurement of the humerus is not part of a POCUS exam.

Extremities

Extremities are very important to document; both hands and arms, and both legs and feet should be identified.

Images 27, 28. Images of fetal extremities.

Fetal sex identification

Many women and their families want to find out the baby’s sex when you are doing a scan. It is perfectly fine to look for this. Although it seems to have a non-medical or social purpose, there are times when it is important to determine certain genetic risk factors.

Images 29, 30. Images of fetal genitalia.

Fetal Anatomy

Table 2. outlines the components of a basic fetal anatomic survey. Note that these tasks will not be reviewed in this course or series. Although fetal anatomy is NOT part of a POCUS exam, it is presented here for information so that the learner will understand these components to adequately interpret reports of outside ultrasounds.

Table 2. Components of fetal anatomy survey (part of a standard ultrasound exam, not POCUS).

SUMMARY

Second trimester POCUS indications include:

No prior prenatal care

Vaginal bleeding

Determining presenting part

Placentation

Determining fetal viability or demise

Evaluation of fetal well-being or distress (decreased growth or fetal movement)

Determining cervical status (rule out risk of preterm birth)

Essential components of the second trimester POCUS include:

Fetal number and viability

Fetal presentation

Cervix

Placenta

Cord

Amniotic Fluid

Women's Point-of-Care Ultrasound: Second and Third Trimester

Second and Third Trimester Overview