Hi,
Thanks for posting your query.
Yes, a trans-abdominal ultrasound cannot reliably diagnose pregnancies that are less than 6 weeks gestation. However, a transvaginal ultrasound, can detect pregnancies earlier, at approximately 4 ½ to 5 weeks gestation.
The absence of a gestational sac at 4 weeks of pregnancy can be suggestive of two things:
a.     The sensitivity of a transabdominal scan is low, so we can wait for another 2 weeks to repeat a scan and see if a gestational sac appears. Alternatively, you can get a transvaginal scan done in a week’s time.
b.     If the gestational sac does not appear in another 2 weeks, an ectopic pregnancy needs to be ruled out. An ectopic pregnancy is a condition where the gestational sac (which ultimately gives rise to the whole baby) gets implanted outside the uterus, most commonly in the fallopian tubes.

The gestational sac first appears at about 4 week’s gestational age in a trans-vaginal scan, and grows at a rate of about 1 mm a day through the 9th week of pregnancy. A gestational sac is sometimes seen as early as during the 4th week of gestation in Trans -vaginal scanning, it may not be seen until the end of the 5th week.
A dominant follicle is a follicle which releases an ovum, or egg, in an event called ovulation. After ovulation, the remains of the dominant follicle in the ovary become a corpus luteum cyst; this cyst produces progesterone to maintain a pregnancy.
A dominant follicle that is currently been seen on an ultrasound can be either a corpus luteum cyst (the exact differentiation between these structures can be done only on a Doppler) or an ectopic pregnancy.
I would suggest that the scan be repeated in another 2 weeks so that we can know the exact condition and further management can be planned accordingly.
Hope I have answered your query. I will be available for follow up queries. Please accept my answer in case you have no follow up queries.

Wish you good health and speedy recovery.

Regards,

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