In this article, you can read about the Mean Sac Diameter calculator which is designed to calculate the dimensions of the uterus for use in determining fetal size. In fact, it calculates the uterus size by inputting the length and width. When the calculator calculates the volume of the uterus, it indicates whether the fetus is large, average, or small. It is an easy way to determine the size and weight of a fetus for pregnant women immediately, but unsure of the right date when pregnancy started. The calculator is also designed to be used by health professionals, surgeries, midwives, nurses, and pregnant women.

Take a look other related calculators, such as:

What is a gestational sac in pregnancy?

Gestation is the period during which an embryo or fetus develops inside the uterus. The term gestation usually refers to the period of approximately nine months that a baby spends in the uterus before birth. It is a time of major changes in the body of the mother. On the other hand, there are more changes in the body of a woman during pregnancy than at any other time in her life. Finally, we can say that gestation is a time of anticipation and excitement for parents-to-be.

The gestational sac is a noticeable secondary indicator on ultrasound in early pregnancy. This vital companion of the baby takes over its blood production for a while before it slowly disappears from the 10th week of pregnancy. It is considered normal when it is not accompanied by cramping, bleeding, or other symptoms.

The first sign of intrauterine pregnancy that we recognize on ultrasound is primarily a circular, hypoechoic structure called a “fruit bag“ in ultrasound jargon. The embryo creates the amniotic sac. Strictly speaking, it is a “trap” formed by the mother’s endometrium, in which pregnancy can develop. The so-called decidual reaction is diagnostically essential, which is a very reliable sign of a local pregnancy.

The following structure we can recognize a few days later is really of embryonic origin: the yolk sac. The yolk sac is still recognizable on ultrasound consists of 3 layers of ectoderm, mesoderm, endoderm. The yolk sac’s regression process begins from the 10th week of pregnancy. After the 12th week of pregnancy, you can rarely see the yolk sac. The sac may occasionally persist in the second trimester and remain recognizable on ultrasound, but this has no detrimental effects on the pregnancy. In fact, in an 8 mm bag, we should be able to recognize the contours of the yolk sac using a suitable ultrasound device.

How to measure the gestational sac?

The gestational sac is a food source for the fetus until the 12th week of pregnancy. Only around the 7 weeks will the embryo be seen in the yolk sac, and we can read the heartbeat. The volume of the gestational or embryonic sac in the fifth week of pregnancy is 2-3 milliliters.

Determining gestational age and determining fetal size should be done in women who are unsure of the duration of the last menstrual cycle. Therefore, we can determine the approximate date of conception and birth when we choose this. Childbirth usually occurs around the fortieth week from the first day of the last menstrual period. Only 3-5% of babies are born at such a calculated date of birth. For this reason, some tables and curves compare a child’s gestational age. The values ​​in the mean sac diameter gestational age table are average, which means that the deviations are pretty standard. Meanwhile, if you need to calculate an average for some other values, you can also use our Average calculator.

For the purpose of determining the duration of pregnancy it is necessary to measure: femur length (FL), biparietal diameter (BPD), transthoracic diameter (TTD), abdominal circumference (AC), and Crown-rump length (CRL). The actual childbirth occurs between the 38th and 42nd week of pregnancy.

How to calculate mean sac diameter?

To calculate the mean diameter of the bag, we can use the formula below:

Md = \frac{l+h+w}{3}

Md – middle diameter of the bag
l – length
h – height
w – width

Finally, it is important to say that the length, width, and height of the pregnancy are expressed in the same units.

To estimate gestational age, we can also use the average bag values from the mean diameter bag diagram of gestational age or with the formula:

Pd = M_{bd} + 30

Pd – Pregnancy days
Mbd – Mean bag diameter (in mm)

Specifically, the mean diameter of the sac increases by 1 mm per day, but there may be variations in growth rate. The accuracy of measuring the average diameter of the bag is about +/- 5 days.

Mean sac diameter gestational age table

Below, you can find the average gestational age is determined based on the mean diameter of the sac.

Mean sac diameter (mm)Gestation (weeks)Mean sac diameter (mm)Gestation (weeks)
Mean sac diameter gestational age table

How to use the mean sac diameter calculator?

Once you have determined the length, height, and width of the gestational sac determined by ultrasound, enter all the values ​​in the calculator. The calculator will automatically calculate the mean value. You will find the estimated gestational age based on the determined value using the embedded mean diameter diagram of the sac. To estimate gestational age in weeks, the calculator will turn the day into a week or a month by changing the time units.


1. What does an empty gestational sac indicate?

The amine cavity may be empty at the beginning. If the doctor determines that it is blank on the ultrasound examination, it does not necessarily mean that the pregnancy is over. Such phenomena happen when it comes to early pregnancy, so the ultrasound does not show the seed. On the ultrasound, we can detect the baby around the 5th week. Additionally, there can be an error in the calculations, so if you have calculated that you are in an advanced stage, it is possible that your gynecologist did not accurately record your cycle, therefore, giving the time of pregnancy incorrectly.

2. Where in the uterus is the gestational sac exactly located?

The fondus are the part of the uterus in which the gestational sac is located. The fundus is located above the cervix (uterine opening).

3. Is mean sac diameter the only way to date of pregnancy?

There are various ways to calculate the gestational age of a fetus, including menstrual history, clinical examination, and ultrasound. Of these three, ultrasound is better for determining pregnancy. The age of menstruation is calculated from the first day of the last menstruation. Conceptual age is calculated from ovulation. Gestational age is calculated based on the theoretical time of ovulation, to which another 2 weeks are added.

4. How do you find the mean sac diameter?

The mean sac diameter is measured by summing the gestational sac’s height, length, and width and dividing the results by 3. It is expressed in millimeters.

5. What is a mean sac diameter?

An ultrasound scan gives us the Mean Sac Diameter (MSD). It is the structure in which the embryo is located is measured, i.e., the gestational sac in which the developing embryo is located in very early pregnancy. The gestational sac can be seen on transvaginal ultrasound for the first time around 5 weeks of pregnancy.
On the whole, the first visible element in pregnancy is the gestational sac and can be used to assess pregnancy viability, diagnose early abortion, and estimate gestational age.