Estimating Conception Date from Early Ultrasounds: The Missing Two Weeks

by Jane Foley

First please note that with the human body there are no EXACT answers when it comes to anything. The dating of a pregnancy is an estimated date gathered from averages of thousands of measurements taken within a cross section of our country.

Unless you have undergone a special procedure such as artificial insemination or in vitro fertilization, it is difficult to know the exact date of conception.

Sperm can live at least five days in a woman's body. Some literature extends the life of sperm to seven days. This means that you may conceive days after you've had intercourse.

Dating starts on the first day of your last period. Any time a pregnancy is dated, whether it is from the LMP or an ultrasound, an two extra weeks are always included in the calculation. During the first two weeks of "pregnancy" your body is really preparing for pregnancy. So if your period was on the first of the month, on the 14th, even though this is the "day of conception," you are considered two weeks pregnant.

So how do you find an estimation of when you conceived? Start on the date you had your earliest ultrasound. Take this date along with the weeks and days given by the ultrasound. Now count back to week two. For example, if you had your ultrasound on June 13th which dated you at 6w5d, you would count back 4w5d and find May 11th conception. See the chart below for examples:

Date of Ultrasound Pregnancy Dating Conception Date
June 13th 6w5d May 11
June 20th 7w5d May 11
June 29th 9 weeks May 11

Just remember -- The most common mistake when trying to determine the day of conception is counting all the way back to day 1! You will be two weeks off if you make this miscalculation.

Jane Foley has worked as a Sonographer (Ultrasound Technologist) for over twenty-three years. Being somewhat of a gypsy, Jane has lived and worked in many parts of the world. She pulls a wealth of information from her experience in the field of Radiology and her interactions with such a broad cross-section of cultures. She now makes her home on the island of Maui with her English husband, Michael.

Copyright © Jane Foley. Permission to republish granted to Pregnancy.org.