Five weeks is an early point in a pregnancy, with the gestational age calculated from the first day of the last menstrual period (LMP). At this stage, the developing structures are microscopic. An ultrasound performed at this time provides a foundational view of the pregnancy’s location and earliest elements. The information gathered confirms the pregnancy’s presence in the uterus, but the visibility of the embryo itself is often limited.
Why Timing and Scan Type Matter
The five-week measurement assumes a typical 28-day menstrual cycle, meaning fertilization occurred only about three weeks prior. This two-week discrepancy explains why the structures viewed are so small. A slight error in recalling the last period or a variation in ovulation timing can significantly change what is expected on the scan.
To visualize these minute structures, a transvaginal ultrasound (TVUS) is almost always required at five weeks. This method involves inserting a specialized wand into the vagina, allowing the transducer to be placed closer to the uterus. An abdominal ultrasound often lacks the necessary resolution to reliably capture anything at this stage.
The Earliest Signs: Gestational and Yolk Sacs
The primary structure seen at five weeks is the Gestational Sac (GS), which is the first sonographic evidence of an intrauterine pregnancy. It appears on the scan as a small, fluid-filled black circle nestled within the uterine lining. The presence of the GS confirms the pregnancy is located in the correct area, reducing the concern for an ectopic pregnancy.
The Gestational Sac expands rapidly. At five weeks and a few days, the Mean Sac Diameter (MSD) can range from approximately 5 to 13 millimeters. Shortly after the GS appears, the Yolk Sac (YS) becomes visible inside it, looking like a smaller, bright ring. The yolk sac is a temporary structure that provides the developing embryo with essential nutrients before the placenta takes over this function.
Addressing the Heartbeat Question
The most common question at this early scan concerns the heartbeat, and its absence at exactly five weeks is typically not a cause for immediate concern. The Fetal Pole (FP), the first visible sign of the embryo itself, is often just beginning to form as a thickening on the margin of the yolk sac. It is usually too small to measure reliably or may not be visible at all.
Cardiac activity is generally expected to appear when the fetal pole reaches a length of 2 to 4 millimeters, which usually occurs around 5.5 to 6 weeks of gestation. While a flicker of the heart can sometimes be detected at five weeks using a transvaginal approach, it is not consistently seen until the pregnancy is more advanced. The medical guideline for diagnosing a non-viable pregnancy is based on the absence of a heartbeat once the fetal pole measures 7 millimeters or more.
What an Inconclusive Scan Means
If the ultrasound is performed at exactly five weeks and does not show the expected structures, it is often labeled “inconclusive.” The most frequent reason for this is a simple dating error, meaning the pregnancy is not as far along as calculated by the last menstrual period. This is especially true if a gestational sac is not visible when the pregnancy hormone level (hCG) exceeds 2,000 mIU/mL.
When the scan is inconclusive, the next step usually involves a follow-up plan, including blood tests to track the rise of hCG levels. The physician will typically schedule a repeat ultrasound in seven to ten days to re-evaluate the structures, confirm progression, and rule out complications like an ectopic pregnancy.