Who is the biological father?

In the situation described, most likely the only way to determine paternity will be to perform DNA tests. Ovulation, i.e. the release of an egg, occurs around the 14th day of the cycle (in the case of a 28-day cycle, for longer cycles this day will move to about 14 days before the next menstruation). Ovulation usually lasts 1 day (24 h). Already at this stage, fluctuations may occur - due to stress, diet, change of residence, use of stimulants, etc. - the time of ovulation may change (the release of the egg may occur earlier or a few days after the calculated date). Sperm, under favorable conditions, can "survive" in the genital tract up to 4-7 days. So fertilization can occur even when intercourse took place a few days before ovulation. For these reasons, it is assumed that the fertile days in women occur more or less between the 7th and 15th day of the cycle. Pregnancy tests are based on the presence of a hormone called chorionic gonadotropin in the blood. It is produced by an embryo that will implant in the uterus. The time of implantation (from fertilization in the genital tract) is about 6-12 days - during this time the embryo "wanders". Pregnancy tests performed 10 days after intercourse may come out falsely negative - the fertilized egg may not have had time to implant during this time (it is enough that ovulation occurred 3-4 days after intercourse, and sperm were still present in the fallopian tube). https://www.gdziepolek.pl/kategorie/test-ciazowy For these reasons, it is impossible to determine who the biological father might be.