The first trimester, mostly defined as the first 100 days of pregnancy, is characterized by many important landmarks heralding the ultimate outcome of pregnancy. Woman becomes aware of her pregnancy after missing her period, being already two weeks postconception at that time. A positive pregnancy test opens Pandora's Box, raising more questions than giving answers. Although a positive pregnancy test most likely suggests an intrauterine pregnancy, production of human chorionic gonadotropin (hCG) occurs as well in tumors (dysgerminoma, choriocarcinoma) or maldeveloped pregnancies, such as ectopic pregnancy, blighted ovum or mola hydatidosa. Other early pregnancy complications and failures, like subchorionic hematoma, missed abortion, incomplete miscarriage, retained products of conception, are likely to be accompanied by clinical symptoms such as lower abdominal pain and/or vaginal bleeding, and suboptimal beta hCG serum levels. Transvaginal ultrasound probes with frequencies of up to 14 MHz have lowered the threshold for US-detection of intrauterine pregnancy to 1200 mIu/ml beta hCG/serum (discriminatory zone), and enable identification of all above-mentioned 1st trimester pregnancy disorders earlier than ever before. Furthermore, the additional interrogation of the region of interest (ROI) with color Doppler (CD) and pulsed-wave Doppler (PW) supplies important information about characteristics of vascularization and flow indices, which assists in further differentiation and prognosis of abnormal early pregnancy findings. With the introduction of transvaginal three-dimensional (3D) sonography, and real-time 3D ultrasound (4D), in vivo studies of the early fetal life became possible. The developmental progress of the embryo and early fetus, its anatomy, and first movement patterns, have been explored by means of ultrasonic 3- and 4D imaging, which can be considered as nonteratogenic as long as investigators adhere to certain safety rules. The new field of sonoembryology has emerged, and researchers are penetrating the mists hiding the beginning of human life. Another area of remarkable expansion has been the 1st trimester scan between 11 and 13/6 weeks of gestation. It includes not only the early diagnose of fetal structural anomalies, like acranius-anencephalus sequence, and the screening for fetal aneuploidies such as trisomia 21,18 and 13, but also offers likelihood ratios for hypertensive pregnancy disorders (pre-eclampsia) and intrauterine growth restriction (IUGR).