An ultrasound at five weeks occurs shortly after a missed period and a positive pregnancy test. This early scan offers the first visual confirmation that a pregnancy is developing inside the uterus. While the structures visible are incredibly small, the examination provides important initial information about the location and progression of the pregnancy. Findings at this stage are measured in millimeters, making the timing of the scan highly sensitive to minor differences in development.
Types of Early Pregnancy Ultrasound
To visualize the tiny structures of a five-week pregnancy, medical professionals rely on a transvaginal ultrasound (TVU) approach. A transvaginal scan utilizes a high-frequency probe inserted into the vagina, positioning it much closer to the uterus and pelvic organs. This proximity is necessary because the early developing pregnancy is deep within the pelvis and microscopic in size.
The alternative, a transabdominal ultrasound (TA), involves placing the transducer on the abdomen. At five weeks, sound waves from a transabdominal scan must travel through the abdominal wall and other tissues, resulting in reduced image clarity. The transvaginal method provides superior resolution and detail, which is necessary to confirm the presence and location of structures measured in millimeters.
Expected Findings at 5 Weeks
The primary structure typically visible at five weeks is the Gestational Sac, which appears as a small, dark, fluid-filled circle within the uterine lining. This sac is the first visible sign of an intrauterine pregnancy and serves as the protective home for the developing embryo. At this stage, the gestational sac usually measures about five millimeters in diameter.
A second structure, the Yolk Sac, is often visible inside the gestational sac by the end of the fifth week, appearing as a small, brighter circle. The yolk sac provides initial nourishment to the embryo before the placenta takes over this role. Its presence is a positive indicator that the pregnancy is developing in a healthy manner.
The mean sac diameter (MSD) can be measured to estimate gestational age, with the sac growing approximately 1.13 millimeters per day. Seeing both the gestational sac and the yolk sac confirms the pregnancy is correctly implanted within the uterus. This early identification is particularly helpful in ruling out an ectopic pregnancy.
Understanding Variability and Dating
The five-week mark is often highly variable, and it is not uncommon for expected structures to be absent during the initial scan. The most frequent reason for this is inaccurate dating of the pregnancy. Gestational age is typically calculated from the first day of the last menstrual period (LMP), but this calculation assumes ovulation occurred exactly two weeks later.
If ovulation or implantation was delayed by only a few days, the pregnancy may be developmentally closer to four and a half weeks. A small difference in development time can dramatically change what is visible, as the structures are growing rapidly and are still extremely small. For instance, the gestational sac is typically only visualized once the human chorionic gonadotropin (hCG) hormone level reaches between 1,500 and 2,000 mIU/mL.
When the findings are inconclusive, a follow-up scan is routinely recommended, usually scheduled seven to ten days later. This delay allows enough time for the expected growth to occur, providing clarity and confirmation of a healthy progression. An early scan that shows nothing conclusive often warrants patience rather than immediate concern.
What Comes Next: The 6-Week Milestone
If the pregnancy progresses normally, the six-week mark brings the next developmental milestones visible on ultrasound. The Fetal Pole, the first visible sign of the embryo itself, typically becomes detectable around this time. It appears as a tiny, slightly curved structure adjacent to the yolk sac.
Once the fetal pole is visible, the next anticipated event is the detection of cardiac activity. High-resolution transvaginal ultrasound can usually pick up the rhythmic pulsing of the embryonic heart tissue around six to six and a half weeks. This early heart rate can range between 90 and 110 beats per minute and is a significant indicator of viability. Seeing the fetal pole and detecting the heartbeat provides reassurance and confirms that development is proceeding as expected.