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Diagnosis of early pregnancy
Last reviewed: 03.07.2025
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Visualization of the fertilized egg is the first sign of early pregnancy. Most often, the fertilized egg is detected after 5 weeks of amenorrhea and is often located asymmetrically in the uterine cavity.
Any normal pregnancy should be detected no later than 6 weeks by the presence of a "double echogenic ring" in the uterine cavity. The inner ring is of a homogeneous hyperechoic structure, 2 mm or more thick. Around it there is a thin, unclosed hyperechoic ring that does not completely surround the ovum. Between the two rings there is an anechoic residual uterine cavity.
At 5-6 weeks, the largest diameter of the fertilized egg is 1-2 cm. At 8 weeks, the fertilized egg occupies half of the uterus: at 9 weeks, it occupies 2/3 of the uterus, at 10 weeks, it occupies the entire uterus.
The gestational age is determined with an accuracy of 1 week by the average diameter of the ovum. On a longitudinal section, measure the maximum size along the length (length), at an angle of 90° - the anteroposterior size (AP). Make a transverse section at a right angle to the longitudinal plane and measure the largest size of the width of the ovum. The average diameter of the ovum is determined as the arithmetic mean of three sizes.
Average diameter of the fertilized egg = Length + Anterior-posterior size + Width / 3
The gestational age of the fetus can also be determined using standard tables.
Intrauterine contraceptive devices (IUDs)
Are there intrauterine contraceptives?
Ultrasound is an ideal method for determining the position of the intrauterine contraceptive in the uterine cavity - whether the IUD is positioned correctly or has fallen out of the uterus.
If the patient is sure that she has an IUD, but it is not visualized in the uterine cavity or in the pelvic cavity, it is necessary to examine the entire abdomen. The IUD can be determined anywhere, up to the splenic angle. If there is any doubt, it is necessary to conduct an X-ray examination (in this case, pregnancy must be excluded) of the entire abdominal area, from the diaphragm to the pelvic floor.
IUD and normal pregnancy
If the IUD is located at a significant distance from the site of attachment of the fertilized egg, then the pregnancy can develop normally.
If the IUD partially falls out, pregnancy can also develop without complications.
If the "whiskers" of the IUD are visible in the vagina, you can try to carefully remove the IUD.
In all other cases, spontaneous miscarriages are possible, and the patient must be warned of this possibility.
Ectopic pregnancy
In the case of an ectopic pregnancy, the fertilized egg may be visualized outside the uterine cavity. Sometimes, a structure resembling a fertilized egg is detected in the uterine cavity, despite the fact that the pregnancy is ectopic. A true fertilized egg can be distinguished from a false one by the presence of parts of the embryo, a yolk sac, and a double ring, while a false egg has a single hyperechoic ring.
Embryo
Although the fertilized egg can be determined sonographically from 5 weeks, and in some cases from 6 weeks of pregnancy, in most patients the embryo begins to be visualized only from 8 weeks of pregnancy. The embryo is determined as a hyperechoic structure located eccentrically in the cavity of the fertilized egg. If the embryo is alive, then the heartbeat is determined in the middle part, in the projection of the chest, closer to the front.
After 9 or 10 weeks, the fetal head can be differentiated from the body, and movements of the embryo can be determined. From 10 weeks of pregnancy, the fetus becomes increasingly similar to a human. After 12 weeks, the skull begins to be visualized.
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Yolk sac
From 7 weeks of pregnancy, a round cystic formation of 4-5 mm in diameter can usually be seen next to the fetus. This is the yolk sac, the place where the primary blood cells are formed. The yolk sac disappears at about 11 weeks of pregnancy. The yolk sac may not be visualized even in a completely normal pregnancy.
It is important to see the cystic cavity in the yolk sac and not to confuse it with the second embryo - a twin. (The yolk sac should not be included in the crown-rump length measurement.)
Multiple pregnancy
Multiple pregnancy is diagnosed from 8 weeks of pregnancy; however, not every gestational sac that is detected contains an embryo. Never tell a patient that she has multiple pregnancy until two or more normally developing embryos are clearly visible. This can be determined around 14 weeks of pregnancy, but is best determined between 18 and 22 weeks.
Multiple pregnancies are usually recognized at around 8 weeks, but the patient should not be told about this until a repeat examination after 14 weeks.
If multiple pregnancy is suspected early in pregnancy, use longitudinal scanning. Abdominal muscles can create artifacts that simulate multiple pregnancy (lens effect).