My ultrasound said the baby measured 8weeks & 4 days (May 29 conception date). But I don't understand how because I didn't have sex that day or anytime before. I had sex May 31st-June 4. so how is the baby measuring that big?

What you don't understand.. is that is a VERY rough estimate. Actual conception time can be up to two weeks before to two weeks after that estimate.

That means it isn't pegged on ONE day... it actually could have been 28 different days.

As was said, unless you had certain medical intervention to become pregnant, the exact day of conception won't be known. Do you recall your last period? About May 15? Often, if baby measures close, your pregnancy will be counted from your last period, which often will add a couple of weeks gestation even if you weren't actually pregnant for part of the two weeks. If baby is measuring more than about a week off from your last period, or you don't recall your last period, the measurements will be used to determine gestation (how far along the pregnancy is). When was the first scan? Have you had any since? Do you have concerns about your due date?

An early ultrasound, which are most often used for dating purposes, is usually fairly close... within several days, as babies will develop along a predictable track in the beginning. For example, what can be seen at 7 weeks, will be different from what was seen at 6 weeks. Often just days can make all the difference in what is picked up on an early scan. The dates that you were given, were not far off from the May 31-June 4 time frame.

May 29 to May 31 is only a couple days, As everyone else has said, this is not exact, this is average measurement comparison. Dates will change as noted with each scan.

So unless there is a paternity issue, never worry about conception dates