The time span for the first trimester is from conception up to 12 weeks of gestation. The most accurate way to calculate the length of pregnancy is by knowing the exact date of conception. The first trimester ultrasonography is mainly performed for the confirmation of pregnancy and to assess whether its intrauterine or ectopic or molar pregnancy, first trimester vaginal bleeding or pain abdomen, to determine the gestational age, to assess multiple gestations and its chorionicity, to assess fetal viability, as an adjunct to invasive procedures like chorionic villus sampling (CVS) and fetal reduction, to evaluate adnexal masses or uterine abnormalities. Normal sonoembryology, first-trimester ultrasound, multiple pregnancy, first-trimester complications, ectopic pregnancy, heterotopic pregnancy, molar pregnancy, and risk factors for early pregnancy failure are the important topics which are discussed throughout this chapter.