After ovulation is passed, you cannot get pregnant. Your luteal phase is when you experience "premenstrual" symptoms.
In order to determine your luteal phase, you need to track your basal body temperatures during the month. Take your temperature right when you wake up in the morning at the same time everyday (within an hour). Do not get out of bed (even to go to the bathroom) when you take your temperature. You will observe that before ovulation, your temperatures stay in a "low" range. After ovulation, you will notice your temperatures in a new "high" range. My low range is between 97.0 and 97.9. My high range is between 98.1 and 98.4.
The day you observe the thermal shift you can start counting as day 1 of your luteal phase. Once your period starts again, you count the day before as the last day of your luteal phase.
If things are normal, your luteal phase can be between 10 to 16 days. Usually it is between 10 to 14 days. Your luteal phase needs to be a minimum of 10 days in order for you to be able to sustain a pregnancy. Your luteal phase is not supposed to vary, but I have been tracking mine for over a year, and it has varied a little bit (between 12 and 13 days -- I had one weird cycle in which it was actually 10 days).
You may have a luteal phase defect if you have a short luteal phase, which may be indicative of a progesterone deficiency. If you have been tracking your cycles and are coming up with less than 10 days for your luteal phase, you should consult your health care provider. If your luteal phase is too short, then a fertilized egg may not have sufficient time to embed properly in the uterus before the uterus lining begins to detach for your period. Though this defect will not hinder conception, it will hinder your ability to stay pregnant.
If you are trying to plan for pregnancy, you also need to track your cervical fluid consistency. Your cervical fluid consistency tells you when you are fertile and therefore approaching ovulation. The basal body temperature readings help you confirm that ovulation has passed. Together, they give you a complete picture of your fertility cycle.