This study aims to improve the safety of spinal anesthesia for pregnant patients undergoing elective cesarean delivery. Specifically, the investigators are investigating whether ultrasound measurements of a vein in the groin (the right common femoral vein, or RCFV) can help predict the risk of low blood pressure (hypotension) after spinal anesthesia. The main question it aims to answer is:

Can femoral vena cava collapsibility index predict post-spinal hypotension in pregnant women in left lateral tilt position?

Before receiving spinal anesthesia, participants will undergo a brief and painless ultrasound examination of the RCFV in the groin area while lying in a specific position."

Post-spinal hypotension (PSH) is defined as a systolic arterial blood pressure (SBP) decrease of more than 20% from baseline or an SBP drop below 100 mmHg. This reduction in blood pressure may compromise uteroplacental perfusion, leading to fetal hypoxia and acidosis. PSH is the most common complication in obstetric anesthesia, with an incidence of up to 95% in healthy women. Despite extensive research, the most effective strategy to maintain hemodynamic stability remains under investigation. Various methods, including crystalloid and colloid fluid loading, leg wrapping, head-down tilt, and vasopressor use, have been explored for both treatment and prevention.

The sympatholytic effect of spinal anesthesia induces vasodilation, exacerbating maternal hypotension due to the gravid uterus compressing the inferior vena cava (IVC). This compression reduces venous return and subsequently decreases the IVC diameter.

Current recommendations for term pregnant women undergoing cesarean delivery advocate for a left lateral tilt position to prevent aortocaval compression (ACC), maternal hypotension, and fetal compromise . In the supine position, the IVC is nearly completely obstructed at term up to its bifurcation. However, most women experience only minimal hemodynamic effects due to compensatory mechanisms such as venoconstriction in the lower extremities and collateral circulation via the paraspinal and azygos veins. Clinically significant hemodynamic compromise, known as supine hypotensive syndrome, occurs in approximately 8-10% of term pregnancies, likely due to insufficient compensatory responses .

The right common femoral vein (RCFV), a continuation of the right external iliac vein, is a tributary of the IVC. Because the RCFV is superficially located, it can be easily visualized using a high-frequency ultrasound probe. Importantly, the RCFV is situated distal to the site of aortocaval compression, making it a potential surrogate marker for hemodynamic changes.

Study Hypothesis This study hypothesizes that the peak velocity and collapsibility index of the RCFV in the inguinal region, measured in the left lateral 15-degree tilt position, reflect the degree of aortocaval compression. These parameters may help identify pregnant women at high risk of post-spinal hypotension during elective cesarean delivery.

Inclusion Criteria:

• ASA 2
• Uncomplicated pregnancy
• Height between 150 cm-180 cm
• Signed the informed consent form
• 8 hours of fasting before the operation
• Patients who refuse normal delivery

Exclusion Criteria:

• Obstetric comorbidities affecting caval compression of the aorta
• Transverse development
• Fetal macrosomia
• Uterine anomaly
• Polyhydramnion
• Oligohydroamnion
• Membrane ruptures
• Intrauterine growth retardation
• Mothers with hyperactive lung disease
• Those with autonomic neuropathy
• Kidney failure
• Smokers
• Severe scoliosis or kyphosis
• Multiple pregnancy (twins, triplets,...)
• Those who do not reach T6 sensory block level after 10 minutes
• Those undergoing general anesthesia or IV analgesics for any reason

In this clinical trial, besides the anesthesiologist performing the ultrasonography (USG), the following parties will be masked:

Outcome Assessor(s): The individuals responsible for measuring the post-spinal hypotension outcomes, such as blood pressure and heart rate, will be blinded to the patient's assigned position (supine or left lateral tilt). This ensures that their measurements and assessments are not influenced by the positioning during the ultrasound evaluation.

Data Analysts: The data analysts, who will perform the statistical analysis of the study outcomes, will be blinded to the group assignments (supine or left lateral tilt). This is done to avoid bias in interpreting the results related to the position and its effect on post-spinal hypotension.

Patients: The patients themselves will not be aware of the specific position (supine or left lateral tilt) they are assigned to during the ultrasound procedure, as this will be randomized. However, they will be informed of the pro