How Accurate Are Conception Dates

How accurate are conception dates? Once you find out you are pregnant, you'll immediately want to know when the baby will be due. Determining the due date generally involves estimating the conception date, but this can be tricky.

How accurate are conception dates?

Due Date Accuracy

For women with regular periods, calculating the conception of your fetus is really based on a formula, which causes the due date to be close to the actual date of delivery. For women who have irregular periods, however, determining the date of conception can be difficult to do. How accurate are conception dates? The answer really depends upon the woman and the method of determining the due date. Some doctors rely on the use of an ultrasound to determine the actual due date of the baby. Most doctors, however, choose the initial due date simply based on the first day of your last period.

Determining How Accurate Are Conception Dates

Determining how accurate are conception dates really depends upon how the date is calculated. There are several methods that obstetricians use to predict this date, including the following:

• LMP-Probably the most common method is to note the first day of your last menstrual period. If you are trying to get pregnant, you will more than likely have this date memorized, but if the pregnancy just happened, you may have to make an effort to pinpoint this day. If you are only able to estimate the day, keep in mind that your conception and due dates will be estimates as well. Some doctors will take the first day of your last period and add nine months and one week to determine the due date. Others will add two weeks. If you know when you ovulated and you know the exact date that you and your partner had intercourse, however, you may be able to more closely pinpoint the conception date. Keep in mind, however, that sperm can live for up to four days, so determining the exact time of conception can still be tricky.
• Ultrasound-One of the most accurate methods of determining when conception occurred is the use of an ultrasound. Ultrasound results can be even more precise if you can factor in the first day of your last period. Keep in mind, though, that as your pregnancy progresses, the accuracy of an ultrasound diminishes. In the first trimester, the accuracy typically falls within a week before or after the factored date. By the second trimester, however, the accuracy jumps to within two weeks, and by the third trimester it moves to within three weeks.
• Assisted reproductive methods-It goes without saying that if you have some type of assisted reproductive method, such as in vitro fertilization, you'll know when your conception date is.

Determining Gestational Growth

As your pregnancy progresses, your doctor will monitor the growth of your baby. During this time, he may or may not change the due date of your child, and thus he may reevaluate the conception date of the fetus. A full-term gestational age is 40 weeks, but keep in mind that any time between 38 and 40 weeks is considered full term. Figuring the gestational age of the baby can be difficult to do, however, particularly if you don't remember the first day of their last menstrual period or you have irregular periods.

Another factor that comes into play regarding determining conception and due dates is the growth of the baby. Babies don't all come in the same size, and thus a larger baby may influence a doctor to predict an earlier due date. The same premise works for smaller babies as well.

In conclusion, the closer an ultrasound is performed to the actual date of conception, the more accurate it generally will be. Keep in mind that conception actually occurs when the sperm meets the egg and this may not necessarily happen on the day the intercourse takes place.

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