Can implantation bleeding occur three days after intercourse?

Hi,
Welcome to ehealthforum,
The brown discharge that you describe could be suggestive of ovulation bleed, breakthrough bleed, infective causes, presence of uterine/endometrial polyps etc. It is important that you have regular periods if you are trying for pregnancy (regular menstrual cycles suggest that you are having regular and periodic ovulation). If you have been trying for pregnancy (for more than a year) and have not been successful, you should consider getting a complete infertility work-up done for you and your partner to see if there are any underlying causes for the infertility/inability to conceive. There are many factors (ovarian, uterine, hormonal, stress etc) that govern the chances of having a successful pregnancy. Hormonal issues, problems involving ovaries and ovulation, uterine problems, tubal blocks, cervical or vaginal infections, ovarian failure/suppression etc., should be ruled out for you. For your partner, semen analysis should be done to rule out infections, blocks/obstruction and to confirm the quality and quantity of sperms produced. Treatment as needed (based on examination and investigations results) would be started to help you get pregnant. Consider visiting your gynecologist/obstetrician for further evaluation and assistance and be in regular follow-up and monitoring as instructed. Have diet with foods rich in zinc content to ensure better maturation of egg and sperms. Monitor cervical mucus changes and the BBT (basal body temperature) charting to predict your ovulation and time your intercourse for having fruitful results. Have intercourse in positions that allow for deeper penetration for high deposit of semen.
Take care.

Thank you so much for the reply. I understand there could be many causes for the brown spotting and lack of periods. I took another pregnancy test yet today, which was also negative, so I am pretty certain that I am not pregnant, unfortunately. My husband and I have been trying to conceive for almost one year now, so I am anxiously awaiting an appointment with my OBGYN doctor.
Again, thank you.