Pregnancy

Gestational chamber measurements by week of pregnancy

The gestational chamber begins to form at about 4 weeks of pregnancy (starting counting from the date of the last menstrual period) and grows at a rate of about 1 mm per day. Usually, it becomes visible on ultrasound around 5 weeks, although everything depends on a correct dating of the pregnancy.

One of the first indicators that you can have to understand the progress of a pregnancy, in addition to the beta hCG, is represented by the shape and measurements of the gestational chamber , also called sac or gestational sac.

gestational chamber visible on ultrasound is the first sign of a pregnancy.

Gestational chamber: when does it start to form?

The gestational sac begins to form around the 4th week of pregnancy (counting from the date of the last menstrual period) and grows at a rate of approximately 1 mm per day.

Gestational chamber: when can you see it?

Generally, the gestational chamber begins to be seen from the  5th week of pregnancy (ie about 3 weeks after conception), by performing a transvaginal ultrasound.

Furthermore, in the middle of the 5th week , the yolk sac , which is the first visible structure inside the gestational sac, should also appear on the ultrasound .

Sometimes, however, the gestational chamber  may be visible as early as the 4th week of pregnancy , while in other cases it may not be seen until the end of the 5th ( this occurs when beta hCG is below a certain value).

In general, being able to identify the chamber in early pregnancy depends on many factors, including the type of ultrasound machine, the method used (transvaginal or abdominal), and the orientation of the uterus (normal or retroflexed), as well as a correct pregnancy dating .

In fact, if the conception occurred later than the calculations, the gestational sac, the yolk sac and the embryo will be visible later than expected.

At what beta value can you see the gestational chamber?

The gestational chamber becomes visible when the beta hCG value is greater than 1000 mIU/ml.  Sometimes, however, it can be visible even earlier, for a value greater than 500 mIU/ml.

Gestational chamber measurements

The size  of the gestational sac is directly related to the gestational age (i.e. the week of pregnancy). Consequently, by measuring the size of the room it is possible to date the pregnancy with an average margin of error of one or two days maximum .

To calculate the size of the gestational chamber, only the largest diameter can be measured or the  average of three diameters (MSD) with the formula:

MSD= (length + height + depth) / 3 

To date the pregnancy starting from the measurement of the chamber diameter, the following formula is then applied:

Gestational age (weeks) = [Mean chamber diameter (MSD) in cm + 2.543] : 0.702

Table: gestational chamber measurements by week

This table shows the mean gestational sac diameter (in millimeters) and the  corresponding median estimated gestational age (in weeks).

gestational chamber gestational age gestational chamber gestational age
 Diameter (mm) (weeks)   Diameter (mm) (weeks)
10 5.0 30 7.9
11 5.2 31 8.0
12 5.3 32 8.2
13 5.5 33 8.3
14 5.6 34 8.5
15 5.8 35 8.6
16 5.9 36 8.8
17 6.0 37 8.9
18 6.2 38 9.0
19 6.3 39 9.2
20 6.5 40 9.3
21 6.6 41 9.5
22 6.8 42 9.6
23 6.9 43 9.8
24 7.0 44 9.9
25 7.2 45 10.0
26 7.3 46 10.2
27 7.5 47 10.3
28 7.6 48 10.5
29 7.8 49 10.6

 

When can you see the embryo in the gestational chamber?

Usually, the embryo becomes visible in the gestational chamber between the 6th and  7th  week , if the dating of the pregnancy is correct, of course Also at this gestational age, the heartbeat should also be visible .

Once the embryo has appeared, the gestational chamber will no longer be measured to date and understand the evolution of the pregnancy, but the embryo itself.

Dr Kathryn Barlow

Kathryn Barlow is an OB/GYN doctor, which is the medical specialty that deals with the care of women's reproductive health, including pregnancy and childbirth.

Obstetricians provide care to women during pregnancy, labor, and delivery, while gynecologists focus on the health of the female reproductive system, including the ovaries, uterus, vagina, and breasts. OB/GYN doctors are trained to provide medical and surgical care for a wide range of conditions related to women's reproductive health.

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