Evolution of Uterine Artery Doppler Waveform Throughout Pregnancy

In non-pregnant women and early pregnancy, the uterine artery typically shows low blood flow speeds at the end of the heart cycle and a dip in flow during early relaxation. Various factors like heart rate, use of blood pressure medications, menstrual cycle hormones, and conditions like polycystic ovarian syndrome can influence the resistance in the uterine arteries. This resistance is reflected by indices like the pulsatility index (PI) and resistance index (RI)(Figure 3)⁴, both of which are influenced by ethnicity and are lower in women with higher BMI. Researchers have developed reference ranges for these Doppler parameters from the early to late stages of pregnancy. Typically, these resistance values drop as pregnancy progresses due to changes in the uterine vessels.

Pulsatility Index (PI)	Resistance Index (RI)
Measures the variation in blood flow within a vessel	Measure of the resistance to blood flow in a vessel
(Peak systolic velocity - end diastolic velocity) / mean velocity	(Peak systolic velocity - end diastolic velocity) / peak systolic velocity
Normal reference ranges for PI are determined by gestational age, as the fetal hemodynamic state evolves during pregnancy	RI normal values typically decrease during pregnancy as the uterine blood flow increases. Normal (low resistance) RI <0.55
A higher PI suggests increased variation in blood flow, which may be linked to reduced placental blood supply and restricted fetal growth	A higher RI suggests greater resistance to blood flow, which may point to issues with placental blood circulation or restricted fetal growth
Abnormal patterns include: - Continuous high resistance flow during pregnancy - Ongoing notching throughout the pregnancy period - Reversal of diastolic flow, indicating a severe condition during pregnancy	High resistance is indicated by bilateral notches with a RI greater than 0.55 and unilateral notches with an RI exceeding 0.65

Figure 3

A feature known as "notching," common in early pregnancy, appears in up to 64% of normal pregnancies. After 20 weeks, a notch is described by a noticeable drop in blood flow velocity during diastole, though this often uses subjective measures. The frequency of notching decreases up to the 25th week and then remains steady. Early diastolic notching suggests lower blood flow in early relaxation phases and is linked to vessel elasticity. Persistent notching may indicate issues with maternal blood vessel function, while problems with placental development can lead to continuously high resistance in the uterine artery⁸.

Understanding 1st Trimester Preeclampsia Screening: Interpreting Results and Next Steps

This test (Figure 4) helps to classify patients into low or high risk categories for developing preeclampsia later in pregnancy, but it doesn’t confirm if they have the condition. Results usually come back within 2-5 days. LabCorp’s first-trimester screening for preeclampsia looks at a mix of factors: details about the mother's health, two biochemical markers (Placental Growth Factor and Pregnancy Associated Plasma Protein A), and two physical markers (Mean Arterial Pressure and Uterine Artery Pulsatility Index). This method can correctly identify up to 90% of high-risk cases, much better than using just the mother's history and background, which identifies less than 50% (Figure 5)³.

Figure 4

Figure 5

During the first trimester of pregnancy, low levels of PAPP-A and PlGF, combined with elevated MAP and UtAPI readings, can elevate the risk of preeclampsia (Figure 8). Reduced PAPP-A and PlGF levels indicate inadequate placental development, whereas increased MAP and UtAPI values suggest heightened blood pressure and restricted blood flow in the uterine artery (Figure 6)⁶.

Effects on the baby include¹²:

Preterm Birth: When PAPP-A and PlGF levels are low, and MAP and UtAPI levels are high, there’s a higher risk of the baby being born early, before 37 weeks.

Fetal Growth Problems: If the placenta doesn't work well, as indicated by low PAPP-A and PlGF, the baby might grow less than expected and have a lower birth weight.

Mean Arterial Pressure (MAP)²	Mean arterial pressure = diastolic blood pressure + ⁠(systolic blood pressure - diastolic blood pressure)/3	*Link to Preeclampsia* *Increased*	Normal MAP is 60-90 mmHg. Under 90 mmHg is low risk, over 96 mmHg is stage one hypertension risk
Uterine Artery Pulsaltility Index (UtAPI)⁷	Doppler ultrasound assessment of uterine artery blood flow	*Increased*	Higher UtAPI MoM values suggest uterine artery dysfunction, increasing preeclampsia risk and indicating reduced placental blood flow
Pregnancy-Associated Plasma Protein A (PAPP-A)¹²	A protein that affects how well the placenta cells invade the uterus and how the baby cells grow	*Decreased*	Normal levels (between 0.5 and 2.0 MoM) Levels <0.4 MoM, are associated with an increased risk of developing preeclampsia and other pregnancy complications
Placenta Growth Factor (PIGF)⁶	A blood protein vital for placenta health	*Decreased*	PlGF levels ≥100 pg/mL are normal, indicating a healthy placenta

Figure 6

Navigating the Next Steps: A Guide to Positive Preeclampsia Screening Outcomes

If the screening test indicates an elevated likelihood of developing preeclampsia (Figure 7), it suggests that the pregnant individual could be at risk as the pregnancy progresses. Such a result necessitates intensified monitoring and may prompt the initiation of preemptive measures to manage the condition. These measures might include the administration of low-dose aspirin or adjusting the timing of delivery to be earlier than initially planned, should circumstances demand it. While data indicate that a 150 mg dose may offer increased effectiveness, low-dose aspirin is typically distributed in the United States as 81 mg tablets, which constitutes a suitable preventive measure for women identified as high-risk for preeclampsia. Administering 150 mg of aspirin at bedtime has proven beneficial in lowering the risk of preterm or early-onset preeclampsia in women with positive screening outcomes. This approach is bolstered by robust evidence from well-conducted trials and meta-analyses. Aspirin functions by reducing the levels of a specific enzyme in blood platelets without disturbing the body's overall synthesis of critical substances¹⁴. For women with positive screening results, aspirin usage is correlated with minimal side effects for both the mother and the fetus. The screening process can also be adapted for women expecting twins with minor modifications. Consequently, The International Federation of Gynecology and Obstetrics (FIGO) now endorses preeclampsia screening for all women between 11 weeks and 13 weeks plus 6 days of pregnancy¹¹. Figure 7

Figure 8

Evaluating Abnormal Uterine Doppler: Preeclampsia Screening Comparison

Researchers measured the uterine artery blood flow between 11 and 14 weeks of pregnancy and compared it to readings later at 21 to 24 weeks. They found that the uterine artery pulsatility index (PI), a measure of resistance in the vessels, was higher than the 90th percentile in 77% of early preeclampsia cases and 27% of late preeclampsia cases. At 21 to 24 weeks, this elevated PI persisted in 94% of early preeclampsia cases, 74% of late preeclampsia cases, and 37% of women who did not develop preeclampsia. A model that combined maternal factors, first-trimester PI, and the change in PI between the first and second trimesters was able to predict early preeclampsia with a 90.9% accuracy at a 5% false positive rate. This study suggested that focusing additional testing in the second trimester on the 20% of women identified as high risk in the first trimester could maintain the same prediction rate, reducing the number of women classified as high risk. This was confirmed by a similar study conducted by Gomez et al., where uterine artery flow measurements were taken between 11 and 14 weeks and then again at 19 to 22 weeks. They found that higher PI levels and notching in the artery were linked to pregnancy complications like preeclampsia, gestational hypertension, and fetal growth reduction. A high PI above the 95th percentile posed the greatest risk for complications, even if the PI appeared normal later, and both high PI and persistent bilateral notching were significant indicators of potential problems¹⁰.

Navigating Uncertainty: A Case Study on Early Preeclampsia Risk Detection and Outcomes

The case presented highlights an increased risk for preeclampsia, as evidenced by uterine Doppler assessments. In the first trimester, assessments revealed elevated Pulsatility Index (PI) values along with bilateral notching, as depicted in Figures 9 and 10. These findings persisted into the second trimester, with continued high PI values and notching still present (Figure 11). Fetal growth remains uncompromised, with the fetus approaching borderline large for gestational age. However, the patient has experienced occasional episodes of elevated blood pressure and proteinuria, necessitating careful monitoring to ensure the well-being of both mother and child. As of 32 weeks gestation, the pregnancy is progressing without further complications.

During the first trimester, low levels of Pregnancy-Associated Plasma Protein A (PAPP-A) and Placental Growth Factor (PlGF) were detected, alongside increased Mean Arterial Pressure (MAP) and Uterine Artery Pulsatility Index (UtAPI) values, contributing to the elevated preeclampsia risk, as shown in Figure 8. The reduced levels of PAPP-A and PlGF suggest inadequate placental development⁶. Concurrently, elevated MAP and UtAPI values indicate potential risk for increased blood pressure and reduced uterine artery blood flow. However, the absence of fetal growth restriction suggests that the placental insufficiency has not reached a level sufficient to significantly impact fetal development. Instead, the restricted uterine artery flow, evidenced by notching and elevated MAP, may be contributing to the patient's episodes of increased blood pressure.

Pulsatility indices (PIs) were checked on both sides, and we calculated the average PI. We also noted whether a notch appeared early in the diastolic phase on both sides. We considered the uterine artery Doppler result as abnormal if both notches were present or the average PI was higher than the 95th percentile. For understanding PI values, we used standard reference charts by age (with the 95th percentile value being 2.6 in the first trimester)¹³.

Figure 9

Figure 10

Figure 11

Enhancing Maternal and Fetal Health: The Future of First-Trimester Preeclampsia Screening

In conclusion, the insights and strategies outlined in this chapter equip healthcare professionals with the necessary tools to effectively incorporate first-trimester preeclampsia screening into clinical practice. The ability to accurately interpret data from uterine artery Doppler assessments and related measurements allows for a nuanced approach to assessing risk. Armed with real-world case studies and practical examples, professionals are better positioned to provide vigilant monitoring and targeted interventions. This proactive stance, including the strategic use of low-dose aspirin among other methods, enhances the prospects for improved maternal and fetal health outcomes. As these evidence-based practices are progressively woven into standard care, the landscape of obstetric care will continue to evolve, highlighting the transformative impact of early detection and comprehensive management in reducing the burden of preeclampsia. The commitment to continuous learning and adaptation ensures that healthcare professionals remain at the forefront of patient care, ultimately leading to healthier pregnancies and safer deliveries.

Advanced Ultrasound Techniques: Uterine Artery Doppler First Trimester Pre-Eclampsia Screening

Chapter 2: Navigating First Trimester Preeclampsia Screening: Understanding Results and Next Steps