Ahhhh... the 1st trimester dating ultrasound. It can be exciting and stressful for both the pregnant person and the sonographer. From the patient perspective, this can cause a high level of anxiety. Especially if the patient is bleeding, cramping, or if they have a history of miscarriages. This high level of uncertainty and vulnerability is difficult for the patient. And we, as sonographers, play an important role whether a pregnancy is viable or not.
So let's learn a little about that early pregnancy:
What is hCG?
HCG (Human Chorionic Gonadotropin) is often called the pregnancy hormone because it is made by cells formed in the placenta, which nourishes the egg after it has been fertilized and becomes attached to the uterine wall. Levels can first be detected by a blood test about 11 days after conception and about 12-14 days after conception by a urine test.
Typically, the hCG levels will double every 72 hours. The level will reach its peak in the first 8-11 weeks of pregnancy and then will decline and level off for the remainder of the pregnancy.
What are the basics when it comes to hCG:
A normal pregnancy may have low hCG levels and still result in a healthy pregnancy.
An hCG level of less than 5 mIU/mL is considered negative for pregnancy, and anything above 25 mIU/mL is considered positive for pregnancy.
An hCG level between 6 and 24 mIU/mL is considered a grey area. Further testing is typically recommended.
A transvaginal ultrasound should be able to show at least a gestational sac once the hCG levels have reached between 1,000 – 2,000 mIU/mL. Because levels can differentiate so much and conception dating can be wrong, a diagnosis should not be made by ultrasound findings until the hCG level has reached at least 2,000 mIU/mL.
The hCG levels should not be used to date a pregnancy since these numbers can vary so widely.
There are two common types of hCG tests. A qualitative test detects if hCG is present in the blood. A quantitative test (or beta) measures the amount of hCG actually present in the blood.
How to measure the crown-rump length (CRL) in early pregnancy
The first true fetal biometric measurement possible is the crown-rump length (CRL). By definition, the CRL is not actually measured from the fetal crown to its rump, but instead the longest linear dimension from the cephalic to the caudal end of the embryo with the fetus in neutral position. In early gestation, between 6 and 9 weeks, fetal posture makes little difference in the CRL measurement.
Beyond 9 weeks, flexion or extension can cause significant discrepancy of the CRL.
How to obtain the mean sac diameter in early pregnancy
This measurement is obtained by taking the average of the measurements of the GS in three planes: coronal, sagittal, and transverse. The MSD is useful early in the first trimester, but loses accuracy when it becomes greater than 14 mm, at which time the fetal pole should become visible. When measuring the dimensions of the GS, calipers should be placed on its borders and care should be taken to avoid including the surrounding decidual tissue.
The Yolk Sac
The yolk sac (YS) first appears within the GS at 5 weeks of gestation, and is frequently the first identifiable structure within the GS. Functioning as the first nutritional and metabolic support for the developing embryo prior to establishment of the placenta, it also offers ultrasonographic confirmation of intrauterine pregnancy.
The YS progresses in size to a usual maximum of 6 mm around 10 weeks, and then regresses until it is absorbed between the amnion and chorion by the completion of week 12–13. Measurement of the YS should be performed by placing the calipers on the innermost border of the echogenic rim.
When is it NOT technically normal?
Findings suspicious for, but not diagnostic of pregnancy failure:
1) Crown rump length of less than 7 mm and no heartbeat.
2) Mean sac diameter of 16-24 mm and no embryo.
3) Absence of embryo with heartbeat 7-13 days after a scan that showed a gestational sac without a yolk sac.
4) Absence of embryo with heartbeat 7-10 days after a scan that showed a gestational sac with a yolk sac.
5) Absence of embryo for 6 weeks or longer after last menstrual period.
6) Empty amnion (amnion seen adjacent to yolk sac with no visible embryo).
7) Enlarged yolk sac (greater than 7 mm).
8) Small gestational sac in relation to the size of the embryo (less than 5 mm difference between mean sac diameter and crown rump length).
Findings diagnostic of pregnancy failure
1) Crown rump length of 7 mm or greater and no heartbeat.
2) Mean sac diameter of 25 mm or greater and no embryo.
3) Absence of embryo with heartbeat 2 weeks or more after a scan that showed a gestational sac without a yolk sac.
4) Absence of embryo with heartbeat 11 days or more after a scan that showed a gestational sac with a yolk sac.
a) Uterine size smaller than the period of amenorrhoea.
b) Cavity filled up with products of conceptions which gave echoes of amorphous masses of different sizes and shapes and echogenicity representing placental tissue, blood clots etc.
a) Demonstrable fetus.
b) Absence of heart activity.
c) Discrepancy of size between fetus and gestational sac.
For TAS, a gestational sac diameter of 20 mm without a yolk sac or 25 mm without a fetus was taken as a criteria of blighted ovum.
For TVS the corresponding figures were 10 and 18 mm, respectively
Threatened abortion was mainly diagnosed clinically as there were no specific ultrasonographic findings but the patient presented with suggestive complaints.
Images and information from:
Papaioannou, George & Syngelaki, Argyro & Poon, Liona & Ross, Jackie & Nicolaides, Kypros. (2010). Normal Ranges of Embryonic Length, Embryonic Heart Rate, Gestational Sac Diameter and Yolk Sac Diameter at 6-10 Weeks. Fetal diagnosis and therapy. 28. 207-19. 10.1159/000319589.
Khanam, Rosy & Nath, Pranoy. (2021). Role of transvaginal sonography in the diagnosis of early pregnancy failure. International Journal of Reproduction, Contraception, Obstetrics and Gynecology. 10. 662. 10.18203/2320-1770.ijrcog20210324.