How to Calculate Conception Without a Period

Figuring out when you conceived can be tricky, even under the best of circumstances. But if you discover you are pregnant when you haven't had a recent period -- because you are breastfeeding or have a low body weight, for instance -- or simply can't recall your last period, determining the date of conception becomes even more difficult. Fortunately, with some detective work and the help of your healthcare provider, you can still figure out roughly when you conceived your baby.

Check the date you first had a positive home pregnancy test. On average, it takes 13.6 days after conception for an test to turn positive, according to a study by the charting website 1. On the date of your first pregnancy test -- you should at minimum -- be about 2 weeks past conception. By itself, this is not a reliable test, however, since you could also be much farther along than 2 weeks.

Measure the size of your uterus with a pelvic exam. Your doctor will feel your abdomen or use a tape measure. The distance from the top of your pubic bone to the top of your uterus, or your fundal height, grows approximately a centimeter every week of your pregnancy. However, this only provides a rough estimate of gestation, since a variety of factors -- including fibroids, multiple babies, the amount of amniotic fluid and the baby's position -- can all influence the measurement. By 28 to 30 weeks of pregnancy, measuring fundal height with tape measure becomes too inaccurate.

Estimate a baby's size with a dating ultrasound, as early as three weeks after conception. A baby's length from head to bottom provides a very accurate estimate of a baby's gestational age until 12 weeks, and is most accurate between and 7 and 10 weeks. During the second trimester, ultrasound measurements still provide a reasonably accurate measurement of gestational age between 12 and 22 weeks. By the third trimester, however, ultrasound estimates of gestational age may be off by plus or minus three weeks.

Calculate your likely conception date, once you have an estimate of your baby's gestational age. Since pregnancy dating usually counts the length of time since the start of the last menstrual period, conception takes place when you are considered 2 weeks pregnant 5. If an ultrasound measures your baby as being 8 weeks 6 days, conception occurred approximately 6 weeks 6 days ago. Simply count back on calendar 6 weeks and 6 days, to find your estimated date of conception.


The earlier you see your healthcare provider, the better your odds of an accurate estimate of your conception date. In the first trimester, estimates of a baby's gestational age are more accurate because only small variations exist in uterine and fetal size. Starting in the second trimester, however, babies begin growing at different rates, making measurements increasingly inaccurate.

If an early ultrasound shows a heartbeat, but the baby is still too small to measure, you are approximately 5 1/2 to 6 weeks pregnant.

If possible, have a trans-vaginal rather than abdominal ultrasound done in the first trimester. Abdominal ultrasounds during this time period may underestimate a baby's age by 1.6 days.