What Is Ultrasound and How Does It Work?

Ultrasound (also called sonography) uses high-frequency sound waves — above the human hearing range — to create images of structures inside the body.[^1][^2] A hand-held device called a transducer sends sound pulses into the body. When the pulses hit tissues of different densities, they bounce back as echoes. The ultrasound machine converts those echoes into a real-time picture on a screen.[^2]

Dense structures like bone reflect most of the sound and appear bright (white) on screen. Fluids such as urine or blood reflect almost no sound and appear dark (black). Soft tissues appear in various shades of grey.[^2]

One of the greatest advantages of ultrasound is its safety: it uses no ionizing radiation, which is especially important for reproductive organs, as ovaries are particularly sensitive to radiation.[^3][^5] The procedure is non-invasive, widely available, portable, and cost-effective compared to MRI or CT scans.[^3]

Key principle:
Higher frequency waves produce sharper images but penetrate less deeply. Lower frequency waves reach deeper structures but with less detail. Doctors choose the right frequency for each clinical situation.[^2]

What Types of Reproductive Ultrasound Exist?

Reproductive ultrasound can be performed in several ways, each suited to a specific purpose. The table below summarises the main approaches.[^4][^5]

Source: Kondagari L et al., StatPearls (2023)[^4]; Dobaria DG et al., StatPearls (2025)[^5]

Transvaginal ultrasound is considered the gold standard imaging technique for pelvic evaluation in fertility assessment because the probe sits close to the ovaries and uterus, producing sharper images than a transabdominal scan.[^4][^5] Both approaches are often used together to get a complete picture.

How Is Ultrasound Used in Female Fertility Assessment?

When investigating female infertility, ultrasound provides a non-invasive window into the reproductive organs. It can assess the ovaries, uterus, endometrium, and surrounding structures — often in a single appointment.[^5][^10]

→ Learn more: Female Infertility

How Are the Ovaries Assessed?

Transvaginal sonography evaluates ovarian size, shape, and the number of small resting follicles known as the antral follicle count (AFC). A lower AFC may indicate a reduced ovarian reserve, meaning fewer eggs are available.[^10] The scan also identifies ovarian cysts — both functional (normal) and pathological (for example, endometriomas) — and helps characterize their size, location, and composition.[^10]

In women with polyendocrine metabolic ovarian syndrome (PMOS), an ultrasound may reveal the characteristic pattern of multiple small follicles arranged in a “string of pearls” within the ovary. The current diagnostic threshold — excluding clinical and hormonal signs — is 20 or more follicles per ovary or an ovarian volume of 10 mL or more.[^10]

→ Learn more: Polyendocrine Metabolic Ovarian Syndrome (PMOS)

How Are the Uterus and Endometrium Assessed?

The uterus is examined for size, shape, and position. Ultrasound can detect fibroids (benign growths in the uterine wall), polyps, and congenital anomalies such as a septate or bicornuate uterus that may interfere with implantation or increase miscarriage risk.[^5][^10]

The endometrial thickness is measured to assess whether the uterine lining is developing appropriately for the stage of the menstrual cycle. A thin or irregular endometrium may raise suspicion for conditions such as intrauterine adhesions (Asherman’s syndrome) or hormonal abnormalities, which require further evaluation.[^5][^9]

What Does Doppler Ultrasound Show?

Doppler ultrasound provides additional information by measuring blood flow to the uterus and ovaries. Adequate blood supply is considered important for follicle development and embryo implantation.[^11] Researchers have investigated the uterine artery and ovarian blood flow indices as potential predictors of IVF outcomes, though the clinical value of these measurements remains an active area of study.[^11]

Can Ultrasound Detect Endometriosis?

Endometriosis — the growth of tissue similar to the uterine lining outside the uterus — is a known cause of infertility. Transvaginal ultrasound can identify ovarian endometriomas (“chocolate cysts”) and, in experienced hands, may detect deep infiltrating endometriosis. A systematic review concluded that no single imaging modality is sufficiently accurate to replace surgical diagnosis for all forms of the disease.[^13]

→ Learn more: Endometriosis

How Does Ultrasound Guide Ovulation Monitoring?

Ovulation monitoring with ultrasound is considered the reference method for estimating ovulation timing by identifying characteristic follicular changes.[^18] Ovulation is the process in which an ovary releases a mature egg (ovum), usually once during each menstrual cycle, making it available for fertilization. Monitoring involves serial transvaginal scans that track how follicles in the ovary grow day by day.

How Does Follicle Tracking Work?

During a natural or stimulated cycle, the doctor measures the diameter of developing follicles every few days. Signs of ovulation include:[^18]

• Sudden disappearance or decrease in the size of the dominant follicle

• Increased echogenicity inside the collapsed follicle (indicating corpus luteum formation — a temporary structure that forms in the ovary after ovulation and produces hormones, mainly progesterone, to support the uterine lining for a possible pregnancy)

• Free fluid in the pelvis

• Transition of the endometrium from a “triple-line” pattern to a homogenous, bright appearance

A recent prospective cohort study of 311 subfertile couples found that the ultrasound-guided ovulation-monitoring group achieved a pregnancy rate twice that of the home ovulation-test group (adjusted OR = 2.12, 95% CI 1.32–3.39), with no evidence of increased emotional distress or impaired sexual function.[^17]

How Is Ovulation Monitored During IVF?

In IVF, controlled ovarian stimulation aims to produce multiple mature follicles. The European Society of Human Reproduction and Embryology (ESHRE) guidelines recommend either AFC or anti-Müllerian hormone (AMH) to predict ovarian response.[^14] Follicle-tracking scans during stimulation typically begin around day 5 of gonadotropin injections and are repeated every 1–2 days.[^9][^15]

A large retrospective analysis of over 9,000 ultrasound scans from 2,322 IVF cycles showed that data from stimulation day 5 were already highly predictive of trigger timing — the point when the clinician gives the ovulation trigger injection during an IVF cycle — and the risk of ovarian hyperstimulation syndrome (OHSS).[^15] This finding supports streamlined monitoring protocols that reduce the number of clinic visits without compromising safety.

→ Learn more: In Vitro Fertilization (IVF)

How Is Ultrasound Used in Male Fertility Evaluation?

Fertility is not only a female concern. Male factors are involved in about half of all infertility cases. Ultrasound is increasingly used to evaluate the male reproductive tract.[^5]

→ Learn more: Male Infertility

What Does Scrotal Ultrasound Reveal?

Scrotal ultrasound with color Doppler is the primary imaging modality for evaluating male reproductive organs. It assesses testicular volume, echogenicity, and blood flow.[^21][^22] The European Academy of Andrology (EAA) has promoted a multicentre study to establish normative scrotal ultrasound parameters using a cohort of 248 healthy, fertile men. The study confirmed that color Doppler ultrasound (CDUS) is useful for detecting abnormalities but noted it still suffers from a lack of standardization and often produces subjective diagnoses.[^19]

A key finding ultrasound can detect is varicocele — enlarged veins in the scrotum — which is the most common correctable cause of male infertility, found in approximately 15–20% of all men and up to 40% of infertile men.[^20] Ultrasound can detect substantially more scrotal abnormalities (such as varicocele or small testicular lesions) than clinical palpation alone.[^20]

A transabdominal ultrasound is usually sufficient as a screening examination for the prostate. Transrectal ultrasound (TRUS) is reserved for more detailed evaluation when specific concerns arise — such as suspected prostate pathology or investigation of ejaculatory duct obstruction.

When Is Transrectal Ultrasound (TRUS) Used?

TRUS evaluates the prostate gland, seminal vesicles, and ejaculatory ducts. It is especially useful for men with obstructive azoospermia (no sperm in the ejaculate due to a blockage). Research suggests that testicular volume may help distinguish between obstructive and non-obstructive causes, though standardized cut-off values have not yet been established.[^22]

What Are the Limitations of Ultrasound?

While ultrasound is remarkably versatile, it is not perfect. Being aware of its limitations helps set realistic expectations.

Source: Koutras A et al. (2023)[^23]; D’Angelo A et al. (2022)[^24]; Thaker N et al. (2023)[^10]

Important:
Ultrasound is one part of a complete diagnostic picture. It works best when combined with clinical history, blood tests (hormone levels, AMH), semen analysis, and, when indicated, additional imaging or surgical procedures.

What Is It Like to Receive Ultrasound Findings?

Ultrasound is not merely a diagnostic tool — it is a human encounter. A recent article introduced the concept of “Sonopsychology,” emphasizing that ultrasound examinations involve complex psychological interactions between patient and examiner, before, during, and after the scan.[^26]

For many patients, especially those undergoing fertility investigations, an ultrasound scan can be emotionally charged. One study of 111 pregnant women showed that anxiety diminished after the ultrasound regardless of the trimester, and that first-trimester scans provided an additional benefit for psychosocial adaptation to pregnancy.[^25]

When findings are unexpected or abnormal, the emotional impact can be significant. A study of 42 Canadian women who received unexpected ultrasound results found that many were not prepared for bad news because they viewed ultrasound primarily as a pleasant opportunity to “see the baby,” rather than as a screening test.[^27] A comparative study at a Swedish university hospital showed that parents with abnormal findings were significantly more worried and anxious than those with normal results.[^28]

What Can Help?

• Ask questions before the scan. Understanding what the examiner is looking for reduces anxiety.[^26]

• Bring your partner or a support person. Having someone with you can help process the information.[^26]

• Request clear communication. Depending on local practice, the examiner or your doctor may discuss the findings during or shortly after the scan.[^26]

So, What Should You Do Now?

If you’re preparing for a fertility assessment or have already received ultrasound results, here’s how to move forward.

Step 1: Ask Your Doctor Which Scans You Need

Not every patient needs every type of ultrasound. Ask your doctor which scans are recommended for your specific situation — transvaginal, transabdominal, Doppler, or a combination.

Step 2: Prepare for the Appointment

For a transabdominal scan, you may be asked to arrive with a full bladder. For transvaginal scans, an empty bladder is preferred. Ask the clinic in advance so you know what to expect.

Step 3: Understand What the Results Mean

Ultrasound findings are one piece of the puzzle. Your doctor will interpret them in conjunction with blood tests, semen analysis (if applicable), and your clinical history. Don’t hesitate to ask for a clear explanation.

Step 4: Follow Up on Any Abnormal Findings

If something unexpected is found — a cyst, fibroid, varicocele, or abnormal endometrium — ask about next steps. Some findings require monitoring, others may need further testing or treatment.

Step 5: Choose the Right Clinic

Ultrasound quality depends heavily on the operator’s experience. Look for a clinic with specialists in reproductive imaging. A second opinion can be valuable, especially for complex findings.

→ Compare fertility clinics worldwide: MedicalNavigator.com/fertility-clinics

Too Long, Didn’t Read

• Ultrasound is safe, radiation-free, and one of the most important imaging tools in fertility medicine.

• Transvaginal ultrasound is the gold standard for evaluating ovarian reserve (AFC), endometrial thickness, and uterine structure.

• Ultrasound-guided ovulation monitoring may almost double pregnancy rates compared to home ovulation kits.

• Varicocele — found in up to 40% of infertile men — is best detected by scrotal ultrasound with Doppler.

• Ultrasound has limitations: it is operator-dependent, cannot reliably detect hormonal causes, and lacks standardized male assessment.

• A study of 311 subfertile couples found no increase in emotional distress from ultrasound-guided monitoring.

References