You’d think figuring out which day you conceived would be simple math, or at least as simple as looking at the calendar, but there are a couple different factors that play into when conception actually occurs. Your menstrual cycle length, ovulation date, and implantation date can all vary.
If you are trying to conceive, you might consider doing a little extra record keeping for accurate dates later on. First, keep track of the days you have intercourse. As unromantic as this sounds, it doesn’t have to be. Keep a roll of stickers by your calendar and place one on each day you try to conceive. You can also mark the date with a pen, either by drawing an “X” or a small heart. Second, keep track of your menstrual cycles. Mark each day that you have bleeding, as well as any other days you notice spotting (which could be implantation bleeding). If you are tracking ovulation, make a note of your suspected ovulation date as well.
If you have been keeping records for a couple months, you should have noticed a trend in cycle length as well as ovulation date. For a rough estimate of when you ovulate, count 14 days back from the first day of your period. Then count from that day back to the first day of the previous period. If your cycle is typically 28 days long, that will come about to be day 14. If your cycle is longer, perhaps 40 days, ovulation will occur around day 26.
By taking your estimated ovulation date and the dates of intercourse, you should be able to calculate which day you conceived. Choose the date on which you had intercourse that is closest to the day you think you ovulated. Conception likely occurred on that day or the day after. Another way to verify is if you experienced implantation bleeding, which typically occurs six or seven days after conception.
An early ultrasound is fairly accurate in determining gestational age. If you have not been keeping track of when you menstruate, ovulate or procreate, an ultrasound is your best bet.