What causes an "implantation dip?"
The term implantation dip is often used to refer to a luteal phase dip that occurs around the time of expected implantation (7-10 days past ovulation). While this pattern does not always result in pregnancy, the term is often used because of the timing of the dip and because this pattern appears with greater frequency on pregnancy charts than non-pregnancy charts. There are a couple of factors that may help to explain why this pattern appears with greater frequency on pregnancy charts.
When you see a mid-luteal phase dip on your chart, however, it does not necessarily mean that you are pregnant. Likewise, you do not have to see this pattern to be pregnant. As long as you have intercourse in your fertile time, you have a chance to be pregnant with any ovulation pattern.
See also a statistical study on this topic.