Gestational chamber (when you see and how much it measures)

Gestational chamber

The gestational chamber is a structure that is created inside the uterus intended to house the embryo and is the first sign visible to the naked eye, by means of an abdominal or vaginal ultrasound, of a pregnancy in progress. The gestational chamber should not be confused with the yolk sac, which is located inside the gestational chamber itself.

Transvaginal ultrasound should be able to detect the gestational sac from the fourth or fifth week of gestational age, when the hGC has a value between 1000 and 2000 IU.

What it can mean if the gestational sac is not visible at week 5

If five weeks have passed since your last menstrual period and your hGC level is not yet known, it is likely that you ovulated late and that your pregnancy has not yet reached the gestational age of five weeks. If, on the other hand, your hCG level is above 1500-2000, and the gestational sac is not visible, an ectopic pregnancy may be in progress.

Finally, if your hCG level falls short of expectations, your doctor will likely order a follow-up ultrasound or continuous monitoring of your hCG level. In that case, a very early miscarriage may have occurred, meaning that the pregnancy stopped developing before the gestational sac had become large enough to be seen on ultrasound.

The gestational chamber grows at a rate of about 1 mm per day until the 11th week. However, there are various factors that contribute to or hinder its visibility (for example, the orientation of the uterus).

In the advanced stages of pregnancy, the size of the gestational chamber allows you to calculate the gestational period with greater accuracy than the LMP method. This can be done using a simple graph, such as the one below.

The alternative to the graphic method, the gestational period can be calculated with a simple formula: (4)Gestational period = (Chamber diameter + 2.543) /0.0702
where: the chamber diameter is expressed in millimeters, the resulting gestational period is expressed in weeks.

The chamber diameter of the gestational sac is the largest diameter of the sac itself (in the photo below indicated with A).


Katherine Johnson, M.D., is a board-certified obstetrician-gynecologist with clinical expertise in general obstetrics and gynecology, family planning, women’s health, and gynecology.

She is affiliated with the Obstetrics and Gynecology division at an undisclosed healthcare institution and the online platform,

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