The first trimester ultrasound scan is one of the main examinations to be performed in prenatal care. With the increase in technology and the development of modern scans it is possible to make diagnoses earlier than before.

The main goals of this particular ultrasound scan are: ascertaining an ongoing intrauterine pregnancy, gestational age evaluation, pregnancy viability analysis, diagnosis of twin pregnancy and its eventual chorionicity, early detection of some gross fetal abnormalities, and early screening for fetal aneuploidies, using nuchal translucency (NT) measurement and other markers.

The gestational sac can be seen using a transvaginal transducer after the 4th week of pregnancy and after 5 weeks, when an abdominal transducer is used. A small anechoic round structure is observed, in the endometrial cavity. At 5 weeks, using the endovaginal transducer, the yolk sac can be observed, which appears before the embryo. The embryo begins to be seen after 5 weeks through a transvaginal scan and 1 week later through the abdominal transducer. When the mean gestational sac diameter measures ≥2.5 cm, it is mandatory to visualize the embryo in a normal ongoing pregnancy. When this does not occur, it is characterized as an anembryonic gestation. The fetal heartbeat can be observed when the crown–rump length (CRL) of the embryo is >4 mm. The presence of the intrauterine gestational sac practically excludes the possibility of ectopic pregnancy, at least for spontaneous pregnancies.

Until embryo visualization is obtained, the gestational age could be calculated using the mean diameter of the gestational sac, although it is not an accurate method of gestational age estimation. The best measure for gestational age calculation, during this period, is the crown–rump length (CRL), which can be measured after 5 or 6 weeks of pregnancy, i.e., as soon as the embryo is seen. This method presents a mean variation of about 5 days (between the 7th and 9th weeks) and 7 days between the 10th and 13th weeks. It is very important to estimate the correct gestational age at this time, as approximately 30% of pregnant women do not remember their last menstrual period correctly or the gestational age is not consistent because of irregular menstrual periods or because of oral contraceptive use, or even because of short inter-pregnancy intervals.

Fetal viability can be ascertained after 6 weeks of pregnancy (CRL = 5 mm) when it is possible to see the embryonic heartbeat. The absence of a fetal heartbeat after this period characterizes a non-evolutive pregnancy, known as abortion. Other ultrasound signs can suggest a bad prognosis for the pregnancy; however, these markers should be carefully analyzed.

The bad prognosis factors related to the gestational sac are the irregularity of the sac, its size in relation to the embryo’s size and a gestational sac measuring over 20 mm without the presence of the yolk sac. When the yolk sac is too big, an association with embryo death is reported. An irregular-shaped yolk sac is also associated with abortion. When the yolk sac is seen, the possibility of anembryonic pregnancy can be excluded.

First-trimester ultrasound can also diagnose multiple pregnancies and their chorionicity. It is important do make a diagnosis of chorionicity because of the increased risk associated with monochorionic pregnancies such as fetal death, twin-to-twin growth discrepancies, and twin-to-twin transfusion syndrome. Before 10 weeks, this diagnosis is carried out by the visualization of the number of sacs. Each gestational sac with its embryo has its own independent placenta. When there is a unique gestational sac with two embryos inside it, a monochorionic pregnancy is diagnosed. After the 11th week, chorionicity is defined by the shape and thickness of the amniotic membrane at the placental insertion. In a monochorionic–diamniotic pregnancy, the amniotic membranes form a T shape at its insertion and two thin layers can be noticed between membranes that separate twins. In a dichorionic pregnancy, the chorion forms the Greek letter lambda (λ).

Diagnosis of first-trimester fetal malformations is also possible, especially using the endovaginal probe. The skull integrity and cranial calcification can be examined after 12 weeks. At the brain, the interhemispheric line and choroid plexuses can be seen. With the normal visualization of these structures, fetal pathological conditions such as anencephaly, acrania, and alobar holoprosencephaly can be ruled out. The diagnosis of encephalocele and ventricular dilatation is sometimes possible.

Heart position can be observed in the fetal thorax. Sometimes it is possible to see the four chambers of the heart. On a transverse image at the fetal abdomen, the stomach and bladder can be evaluated after 11 weeks. In some cases, at the end of the first trimester, both fetal kidneys can be seen. When the stomach is absent, we should repeat the examination and if it persists a diaphragmatic hernia should be hypothesized.

Bladders with significant dilatation should be re-evaluated. Most of the time, spontaneous resolution occurs; however, cases of low urinary obstruction should be suspected when the dilatation becomes worse in the next evaluation. The abdominal circumference can be measured after 12 weeks or when CRL is ≥45 mm. The presence of herniated bowels through the cord insertion leads us to a diagnosis of omphalocele after 12 weeks. The diagnosis of gastroschisis is also possible during this period. The vertebral spine can also be better evaluated after 12 weeks and fetal movements aid a better examination quality.

Spina bifida and rachischisis had been already reported during this period of pregnancy. The superior and inferior limbs can also be examined during the first trimester. In general, arms and hands are settled laterally to the cephalic pole; its visualization can be obtained in a transverse section of the fetus. Hands are usually open and fingers can be counted. Legs and feet can also be seen in a transverse section image. The absence of a limb should be diagnosed during the first trimester of pregnancy. In spite of several reports of bad formation during the first trimester, ultrasound sensitivity is still insufficient to diagnose many malformations and this technique is not a substitute for the second-trimester morphological examination.

The search for ultrasound markers of fetal aneuploidy is a crucial part of the first-trimester ultrasound scan between 11 and 13 weeks and 6 days. Measurement of nuchal translucency (NT) is the most important marker. To achieve the correct measurement of this marker, fetal CRL must be between 45 and 84 mm (11–13 weeks and 6 days). The fetus should be in a neutral position at the sagittal section (the same technique as that used to achieve CRL measurement). The maximum thickness of the hypoechogenic area between the skin and subcutaneous tissue from which the fetal spine evolves (hyperechogenic lines) is measured. It is important to carefully distinguish fetal skin from amniotic membrane, once both have the same image of a thin hyperechogenic membrane. Therefore, we should wait for fetal movements that separate the fetus from the membranes. The image should be zoomed until it takes at least 75% of the frame. The caliper movement should make a change of 0.1 mm at the measurement. At least three different measurements of NT should be made and the biggest one considered. These are the criteria approved by the Fetal Medicine Foundation. The combination of mother’s age, previous history of chromosomopathy, and the NT measurements has a sensitivity of 85% for detecting Down’s Syndrome cases with a 5% false-positive rate. When the software used to calculate the risks is not available, values up to 2.5 mm should be considered normal.

The placenta can also be evaluated at the beginning of the pregnancy with regard to its position and aspect. The presence of a heterogeneous image with many anechoic vesicles of different sizes should be lead to the suspicion of trophoblastic disease. Diagnosis can be made earlier through the endovaginal examination.