: Estimated Date of Conception My girlfriend's estimated delivery date is 23 Feb 2016. Our last intercourse was on 7 May 2015. I checked with some expected conception date calculators on the net. They all say that the conception

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Looks like the mean and standard deviation for gestation are around 280 days and 13 days respectively. Based on the dates you listed, that would put gestation somewhere around 292 days (probably a day or so less as conception is not immediate.) This is less than one standard deviation from the mean. About 16% of pregnancies last longer than the dates you gave. Seems entirely within the realm of possibility.

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Generally, due date is calculated by one of two things.

One, determining the date of the expectant mother's period prior to likely conception. If she knows what days her period started on for the last several periods, this will tend to be fairly accurate.

Two, looking at the size of the fetus (and more specifically, certain particular sizes, such as the length, the head circumference, etc.), and estimating how long it has been gestating for, as well as identifying certain fetal characteristics that happen at certain fairly consistent times. This has varying levels of accuracy depending on when it's used; it tends to get less accurate in the third trimester, but even then the 95% range is 2-3 weeks for most common measurements.

The article "Assessment of Gestational Age by Ultrasound" (MacGregor and Sabbagha, 2008) gives a pretty thorough overview of pre-ultrasound and ultrasound-based measurement techniques, and how accurate they are. In the 12-26 week period (where she would probably have been at the time, and is now), most of the measurements are less than 2 weeks off 95% of the time, and crown-rump length is off by less than a week 95% of the time. Combine several measurements, as they do later in that paper linked above, and the usual error is only a few days.

A side note: None of this should be taken to prove or disprove paternity, as has been noted elsewhere. 95% intervals are just that, 95% likelihood; and this all assumes the doctor and/or tech performing the ultrasound and interpreting it didn't make a mistake. If you're concerned about paternity specifically, there are far more accurate ways of verifying that once the child is born (via genetic testing).