# 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 date is between 25 May to 5 Jun. But my girlfriend insists that it's my child. Please can someone tell me if it is possible that intercourse on 7 May 2015 could have an expected delivery date of 23 Feb 2016?

• Welcome to Parenting.SE. What method did the doctor use to calculate the delivery date -- size of fetus, her last period, or something else? Oct 16 '15 at 15:55
• Doctors use the method is her last date of periods and her periods never comes on time Oct 17 '15 at 5:49
• Also the gestational age of child in ultrasound is 21.3 weeks on 14 of october. Plz tell me if there any chance that i m father of this child Oct 17 '15 at 7:26
• I don't think you're going to be able to get a definitive enough answer from online calculators, estimations, and average fetal growth rates -- how certain do you need to be that you're the father, 100%? In that case, the only possible answer is from a paternity test. Oct 18 '15 at 3:36

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).

• This is why it's important to know whether the delivery date was calculated by counting from date of last period, or from an ultrasound. Calculation can be fairly off if the mother has an irregular or long (> 28 days) menstrual cycle. It is also possible to have light bleeding at implantation or "breakthrough bleeding" which could be mistaken for a regular period; that could throw the calculated estimate off by weeks. Oct 16 '15 at 22:06
• Delivery date was calculated by counting from date of last periods and there was always delay in her periods timings Oct 17 '15 at 5:35

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.

• This is true for the total length of gestation, but this isn't really relevant here: we don't actually know the date of birth yet. The doctor's calculations will be based on other things rather than the length of gestation: in particular, they're using certain details (size of fetus, for example) to determine when the conception likely occurred, and then estimating the due date from that (using 280 days from her last period prior to conception).
– Joe
Oct 16 '15 at 19:42
• According to last ultrasound the gestational age of child is 21.3 weeks on 14 oct. Is there any chance that i m the father of that child Oct 17 '15 at 7:38
• why all the conception calculator on the net is always said that conception date is around 2 june. 293 days of gestation is from the first day of last periods. I m really confused plz some body help me out Oct 18 '15 at 3:13