At what week of gestation can I confirm pregnancy via ultrasound?

Ultrasound Timing for Pregnancy Confirmation

You can confirm pregnancy via ultrasound as early as 5 weeks gestational age using transvaginal ultrasound, when the gestational sac becomes visible, with definitive confirmation at 5½ weeks when the yolk sac appears. 1

Timeline of Ultrasound Findings

Week 5 (Earliest Detection)

• The gestational sac is the first visible structure on transvaginal ultrasound at approximately 5 weeks gestational age, appearing as a round or oval fluid collection surrounded by a hyperechoic rim of trophoblastic tissue 1, 2
• At this stage, without a yolk sac or embryo visible, this should be reported as a "probable gestational sac" or "probable pregnancy" 1, 2

Week 5½ (Definitive Confirmation)

• The yolk sac becomes visible at approximately 5½ weeks gestational age, providing incontrovertible evidence of pregnancy 1, 2
• Once a yolk sac is visualized within an intrauterine fluid collection, the pregnancy is definitively confirmed and should be reported as a "definite gestational sac" or "definite pregnancy" 1, 2, 3

Week 6 (Cardiac Activity)

• An embryo with cardiac activity is typically visualized at 6 weeks gestational age on transvaginal ultrasound 1, 2, 3
• This represents the most reassuring finding for pregnancy viability 2, 3

Critical Technical Considerations

Transvaginal vs. Transabdominal Approach

• Transvaginal ultrasound is the modality of choice for early pregnancy evaluation due to superior resolution and earlier detection capabilities 4
• Transabdominal ultrasound detects structures approximately 1-2 weeks later than transvaginal imaging 5, 6
• Research shows transvaginal Doppler can detect fetal heart rate as early as 6 weeks 0 days, compared to 7 weeks 0 days transabdominally 5

Gestational Age Calculation

• Gestational age is calculated from the first day of the last menstrual period (LMP), not from conception 1
• Mean sac diameter can be used to calculate gestational age before an embryo is visible 1, 2
• Crown-rump length (CRL) becomes the most accurate dating method once the embryo is measurable 2

Important Clinical Pitfalls to Avoid

Misidentifying Intrauterine Fluid

• Never use the term "pseudosac" as it can lead to clinical errors, potentially causing treatment that harms an early intrauterine pregnancy or missing an ectopic pregnancy 1, 2, 7
• Fluid in the endometrial cavity should be described as "intracavitary fluid" or "fluid in the endometrial cavity" based on its characteristics 1, 2, 7

Confirming Signs (Helpful But Not Required)

• The intradecidual sign and double decidual sac sign have high specificity but poor sensitivity and interobserver agreement 1, 7
• These signs can increase confidence in diagnosing an intrauterine pregnancy when present, but their absence does not exclude pregnancy 1, 7

Nonviable Pregnancy Criteria

• Absence of a yolk sac in a gestational sac >8 mm suggests poor prognosis 2, 7
• Absence of an embryo in a gestational sac >16 mm indicates likely nonviability 2, 7
• Definitive diagnosis of embryonic demise requires absence of cardiac activity in an embryo ≥7 mm CRL 2, 3
• For embryos <7 mm CRL without cardiac activity, follow-up ultrasound in 7-10 days is recommended before making a definitive diagnosis 2, 3

Practical Algorithm for Early Pregnancy Ultrasound

At 5 weeks: Expect to see gestational sac only (probable pregnancy) 1, 2

At 5½ weeks: Expect to see yolk sac (definite pregnancy confirmation) 1, 2

At 6 weeks: Expect to see embryo with cardiac activity (viability confirmed) 1, 2, 3

At 7 weeks: Expect to see amnion and more reliable cardiac activity detection 2, 3