What is the normal size of the gestational sac at six weeks?


Gestation refers to the period when the fetus grows and develops in the uterus. It is the time between the conception and birth of the embryo. 

Beginning with fertilization, it ends with the birth of the baby. A gestational period describes the length of the pregnancy period and can range between 38 to 42 weeks.

What is a gestational sac?

A gestational sac is a spherical shaped fluid-filled cavity that is present around the embryo. It is situated in the upper part of the uterus’s fundus. Appearing as an echogenic ring, it surrounds a sonolucent center. 

The availability of this structure within the womb is the only indication of pregnancy, before the identification of the embryo. The gestational sac encloses the baby that is developing. It contains extraembryonic coelom or the chorionic cavity in the early stages of embryogenesis.


The amniotic fluid is also found in it. It is located around a white rim and is a dark space itself.

Structure and Development

The gestational sac is the earliest finding in sonography during pregnancy. Generally located within the uterus, the gestational sac is spherical. It takes around four weeks for the gestational sac to appear. 

It then follows a growth rate of about 1 mm per day for about nine weeks. The size of a gestational sac is determined by the measurement of the largest diameter or by calculating a mean of the diameters.

In the embryogenesis stage, the gestational sac is a part of the extraembryonic coelom. Being a portion of the conceptus, it consists of a cavity between the Trophoblast and Heuser’s membrane. The amniotic sac occupies most of the gestational sac by nine weeks of gestational age.


The gestational sac can be recognized within four weeks of the last menstrual period. With a size of about 2 mm at first, it gradually increases to 5-6 mm by five weeks. Thereby, the average increasing speed becomes one millimeter per day. 

While the yolk sac becomes visible by around the 5th week of the gestation period and attains the size of 6 mm by ten weeks. These yolk sacs disappear by the 12th week on an average.

By the time the fifth week-ends, the fetal pole becomes visible. This is the time when the embryo is first seen. It looks like a tissue nubbin, adjacent as well as distinct from the yolk sac. 

At this time, the length of the embryo is about 2 mm, and it develops along the yolk sac’s chorionic margin. By the sixth week, it looks like a 5 mm echogenic disk which is attached to the yolk sac and is touching and closing tangentially.

In case there is no gestational sac seen in the ultrasound, it can either mean an ectopic pregnancy or a miscarriage. In case the gestational sac is found empty, it can be an anembryonic pregnancy and suggests that the embryo has failed to develop.

With the presence of innovative medical technologies in India, there are some best gynecologist in Chennai, Mumbai, Kolkata and other Indian states. This has made the entire pregnancy process smoother. 

This article gives the reader a brief idea of what a gestational sac and about its structure. It helps in understanding how an embryo develops during the gestational period.


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