is suggestive not conclusive
First let me clarify about the ultrasound reports.
Your last period was on 11/20. The ultrasound was done on 1/15. The gestation or duration of pregnancy
on that day is exactly 8 weeks [56 days] from the date of your last menstrual period. This has been showing on the ultrasound. It is most likely that you ovulated on 2nd or 3rd of January.
I must clarify that ultrasound is used to detect pregnancy and predict dates. It does this with a fair amount of accuracy. There is always a margin of + /- one week in a first trimester ultrasound.
If two people do it on the same patient using different machines, this much difference is expected, If one person does it on two women with similar gestation, still there may be a difference in gestational age
. If one women is tested on two machines on the same day, results may vary. So, for us as obstetricians this difference is expected and understood. This is the reason that paternity cannot be predicted by gestational age of the baby in days if the difference in sexual contact days is so small and very near the ovulation period.
There are two main factors that indicate that pregnancy may have been from the second episode, first is that the contact occurred closer to likely date of ovualtion and also there was ejaculation, whereas the first episode of contact was slightly closer to ovulation and there was no ejaculation. So the chances of pregnancy due to second contact are higher.
About luteal phase: Menstrual cycle has three main phases:
follicular phase; begins form early part just after or the last few days of menses till ovulation.
luteal phase; begins from ovulation till the next menstrual cycle
menstrual phase; is during menses.
Usually the duration of follicular phase varies in females, but the duration of the luteal phase is fixed for 14 days after ovulation. This is the reason that we say that ovulation may occur on any day between 12-18 in a cycle. But the menses will follow 14 days after ovulation. So it may be possible that you ovulated early or late. We can only predict this as we have not seen the actual process of ovulation happening on a certain day by sonography
[ we do this for some women with infertility].
Regarding paternity testing: This is most accurate and gold standard. It actually sees if the chromosomes of father match with those of the baby. Chromosomes contain the genes that we transfer to our children. So they are inherited from the real father only.
I do understand your situation. Please do not worry so much. Unfortunately it is not always possible to determine things without testing. I may speculate and suggest possibilities but the actual result of testing will only help reveal the true situation.
Hope that this helped in satisfying your queries. I am always here to help you out.
Please feel free to ask any more doubts.