At 6 weeks of pregnancy (about 2 weeks from beta hcg test), the surrogate mother will be called at the clinic where she will undergo a transvaginal ultrasound.
The fetal heart activity is among the earliest signs that the pregnancy is viable. The fetal heart rate can be seen by transvaginal sonogram TVS as early as 22 days after fertilization/ovulation or 5weeks 1 day after the last menstrual period. However, many times it’s not seen until 6 weeks and 0 days.
By the time the embryo measures 2 mm the fetal heart rate activity should be seen and normal embryos start at under 85 beats per minute heart rate. In 5-10% of normally progressing pregnancies when the embryos measure between 2 and 4 mm the fetal heart rate cannot be seen, so in and by itself the absence of a fetal heart rate at that time does not mean it’s a miscarriage. By the time the embryo measures 5 mm all embryos should have a heart rate.