Gestational Sac during Pregnancy

Gestational Sac During Pregnancy- What It Indicates

Medically Reviewed By
Dr. Rima Sonpal (Gynecologist/Obstetrician)
View more Gynecologist/Obstetrician Our Panel of Experts

Pregnancy can progress differently for every woman. The conception of a baby can be a complex process. The initial phase of foetal development normally forms the basis for a safe and healthy pregnancy. So, the confirmation of pregnancy is usually followed up by conducting an ultrasound and some blood tests. Your doctor may check the levels of hCG (Human Chorionic Gonadotropin) through a blood test. During an ultrasound, the doctor tries to find the gestational sac, which helps him to ascertain the progress of the pregnancy.

The presence of a gestational sac by three to five weeks of gestation is normally considered a positive mark. It could also happen that a gestational sac is detected but without the presence of an embryo.

In some cases, during the ultrasound, the doctor is unable to see a gestational sac. Such a scenario can arise due to miscalculation of gestational dates, which means it may be too soon to see a gestational sac. But if no gestational sac is spotted even during the follow-ups, it may indicate a possible ectopic pregnancy or a miscarriage.

What Is a Gestational Sac?

The detection of a gestational sac through an early ultrasound is usually the easiest way to gauge the advancement of pregnancy. The gestational sac, which is the cavity containing the yolk sac that surrounds the developing embryo, is usually present in the uterus. It looks like a dark space with a white rim around it when seen on an obstetric ultrasound.

A gestational sac is the only structural evidence available that may indicate the existence of an intrauterine pregnancy until an embryo can be detected. It usually develops about 5 to 7 weeks after the end of the last menstrual cycle. It is normally noticeable by three to five weeks of gestation when the MSD (Mean Sac Diameter) measures about two to three mm in diameter by obstetric ultrasonography or when the HCG levels are around 1500 to 2000 mIU/mL. Additionally, a study conducted in 2013 revealed that hCG levels that help detect the gestational sac were 390 milli-international units/mL, 1,094 milli-international units/mL, and 1,394 milli-international units/mL.

The shape of the gestational sac during the initial days of pregnancy (before eight to ten weeks) is normally important. Although a round-shaped gestational sac is usually considered ideal, there is no definite way of defining what other shapes mean. But in case the doctor finds the gestational sac to be unusually shaped, he may like to monitor it regularly to rule out any probable complications. Preferably, after 1 or 2 weeks of the last examination of the gestational sac, the doctor should be able to detect the fetal heartbeat, which is a sure-shot sign that the pregnancy is advancing well.

What Is Gestational Sac Measurement?

Starting as early as 4.5 weeks into pregnancy, medical professionals can estimate the gestational age by measuring the diameter of the gestational sac. This measurement is known as the mean sac diameter (MSD) and is recorded in millimetres (mm). It’s a reliable method that can be employed between 5/6 and 11 weeks of gestation. However, it is only used until the fetal pole or embryo becomes visible, which is usually around the 6-week mark.

It’s important to note that the rate of sac growth can vary from one woman to another, but on average, it increases by approximately 1.13 mm per day. In a 1987 study, the gestational sac growth of less than or equal to 0.6 mm/day was concluded as abnormal gestational sac development.

Here’s a breakdown of what the size of the gestational sac should be at various gestational weeks:

  • Although it is unclear about the size of the gestational sac at 4 weeks, we know that at 4.5 gestational weeks, the gestational sac size is expected to measure roughly 2-3 mm.
  • The gestational sac at 5 weeks should have grown to approximately 5-6 mm.
  • Similarly, the gestational sac size at 6 weeks typically reaches around 14 mm.
  • Moving on to the 7th gestational week, the sac should measure about 27 mm.

What Is the Normal Appearance of Gestational Sac?

The gestational sac acts like a protective bubble that envelops the developing embryo and is filled with a special fluid called amniotic fluid. When observed on an ultrasound, a normal gestational sac exhibits the following characteristics:

  • It appears as a dark sphere encircled by a delicate, white ring.
  • Its shape is typically round or elliptical.
  • The gestational sac is situated in the upper region of the uterus, known as the fundus.
  • An important sign called the “intradecidual sign” becomes visible before the completion of 5 weeks.
  • Shortly after reaching the 5-week mark, a phenomenon known as the “double decidual sac sign” can be observed, where two small “bubbles” become apparent.
  • Around the 5 to 6-week stage, the yolk sac, which is the initial structure to emerge, confirms the pregnancy’s location in the uterus, where it should be growing safely.

If a Gestational Sac Is Seen on Your Ultrasound, Is It a Guarantee of a Normal Pregnancy?

The discovery of a gestational sac during an ultrasound is surely a positive pregnancy sign. However, the presence of a gestational sac alone may not guarantee a normal pregnancy. Apart from the gestational sac, it is important that a yolk sac, which forms within it, also gets developed. The yolk sac is vital, as the developing embryo draws its nutrition from it till the placenta doesn’t take over. The yolk sac normally becomes apparent by 5 to 6 weeks of gestation on a transvaginal ultrasound. Sometimes, a yolk sac is not found, even though a gestational sac may be visible on ultrasound.

Reasons for Absence of a Gestational Sac

The absence of a gestational sac may be due to the following reasons:

  • The most common reason can be a miscalculation of gestation. You may have to go for another follow-up ultrasound test. Comparing your hCG levels with the results of your ultrasound test can prove helpful here. If your hCG levels are less than 1500, then it may be too early to look for a gestational sac.
  • A miscarriage has taken place or is likely to happen. Dropping hCG levels may be an indication of a miscarriage.
  • If a gestational sac is not visible and the hCG levels are around 1500 to 2000, it may be a sign of an ectopic pregnancy. Ectopic pregnancy or extra-uterine pregnancy means the embryo gets implanted outside the uterus. Such an occurrence is a medical emergency and may require further testing for proper diagnosis and treatment. Serial monitoring of BhCG levels is suggested in this case.

Lack of Gestational SacWhat Does an Empty Gestational Sac Indicate?

It is possible to detect an embryo in the gestational sac at six weeks of gestation. However, sometimes, an anembryonic pregnancy or gestational sac miscarriage may occur. In such an instance, the early gestational sac is empty and does not hold an embryo. It simply means that an embryo was unable to develop. The reasons can range from abnormal cell division to poor-quality eggs or sperm. Such pregnancy loss normally happens during the initial days of pregnancy and often without a woman even realizing that she is pregnant.

In many cases, a woman undergoes natural miscarriage without intervention due to chromosomal irregularities, while in some cases, a woman may have to opt for a D&C (Dilation and Curettage) procedure.

What Your Doctor Will Do if He Sees an Empty Gestational Sac?

An empty gestational sac can be because of reasons like early intrauterine pregnancy, anembryonic pregnancy or blighted ovum. In case your doctor notices an empty gestational sac, he may test your hCG levels and suggest another ultrasound, as sometimes it can be difficult to calculate the gestational age that early in pregnancy, and the follow-up ultrasound tests may reveal an embryo later on.

However, if the results are unfavourable, he may prepare you for a miscarriage, as the pregnancy is not advancing normally and is not likely to result in the birth of a child. The doctor may also declare a pregnancy non-viable (failed pregnancy) if he observes an irregular gestational sac, which simply means that the gestational sac does not possess the well-defined, consistent structure of a normal gestational sac.

FAQs

1. Can twins share the same gestational sac?

Yes, in rare cases. Let us explain. Ideally, identical twins each have their own amniotic sac, however, monoamniotic twins are those 1 per cant cases of identical twin pregnancies that share the same amniotic sac as well as the same placenta. In case of monoamniotic twins, they are at danger of pregnancy complications than other twin pregnancies as they are under the risk of compression or umbilical cord entanglement.

2. Does consuming water help with the increase in amniotic fluid?

Yes, staying hydrated can help with the increase in the amniotic fluid during pregnancy. As per several studies conducted with maternal women, including one in 1993 and another in 2000, it was revealed that simple maternal oral hydration helped increase the amniotic fluid volume. It may also be beneficial in managing oligohydramnios and prevention of oligohydramnios during labour. However, further studies are required to assess the possible risks and benefits of maternal hydration. 

Different women can have varied pregnancy experiences. It is important to remain stress-free and enjoy your pregnancy as much as possible. You can always seek emotional and physical support from family and friends. It is sensible to consult your doctor for the necessary guidance and treatment in case of any complications.

References/Resources:

1. Patience is key: Understanding the timing of early ultrasounds; UT Southwestern Medical Center; https://utswmed.org/medblog/patience-key-understanding-timing-early-ultrasounds/; November 2018

2. Nyberg. D. A, Mack. L. A, et al.; Distinguishing normal from abnormal gestational sac growth in early pregnancy.; Journal of Ultrasound in Medicine; Wiley Online Library; https://onlinelibrary.wiley.com/doi/abs/10.7863/jum.1987.6.1.23; January 1987

3. Suguna. B, Sukanya. K; Yolk sac size & shape as predictors of first trimester pregnancy outcome: A prospective observational study; Journal of Gynecology Obstetrics and Human Reproduction, Volume 48, Issue 3, Pages 159-164; ScienceDirect; https://www.sciencedirect.com/science/article/abs/pii/S2468784718303118?via%3Dihub; March 2019

4. Connolly. A, Ryan. D, et al.; Reevaluation of Discriminatory and Threshold Levels for Serum β-hCG in Early Pregnancy; Obstetrics & Gynecology; https://journals.lww.com/greenjournal/fulltext/2013/01000/reevaluation_of_discriminatory_and_threshold.12.aspx; January 2013

5. Blighted ovum: What causes it?; Mayo Clinic; https://www.mayoclinic.org/diseases-conditions/pregnancy-loss-miscarriage/expert-answers/blighted-ovum/faq-20057783

6. The American College of Obstetricians and Gynecologists; ACOG Practice Bulletin No. 191: Tubal Ectopic Pregnancy; Obstetrics & Gynecology; https://journals.lww.com/greenjournal/abstract/2018/02000/acog_practice_bulletin_no__191__tubal_ectopic.38.aspx; February 2018

7. Puget. C, Joueidi. Y, Bauville. E, et al.; Serial hCG and progesterone levels to predict early pregnancy outcomes in pregnancies of uncertain viability: A prospective study; EJOG; https://www.ejog.org/article/S0301-2115(17)30533-X/fulltext; January 2018

8. Ectopic Pregnancy; University of Michigan Health; https://www.uofmhealth.org/health-library/hw144921

9. Hofmeyr. G. J, Gülmezoglu. A. M; Maternal hydration for increasing amniotic fluid volume in oligohydramnios and normal amniotic fluid volume; Cochrane Database Syst Rev.; PubMed; https://pubmed.ncbi.nlm.nih.gov/10796151/; 2000

10. Kilpatrick. S. J, Safford. K. L; Maternal hydration increases amniotic fluid index in women with normal amniotic fluid; Obstet Gynecol; https://pubmed.ncbi.nlm.nih.gov/8416460/; January 1993

Also Read: 

Yolk Sac in Pregnancy
Foetal Pole on Ultrasound
Viable or Nonviable Pregnancy

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