The luteal phase includes several important events that prepare the body for pregnancy. It starts as the egg makes its way through your fallopian tube and ends with the start of your next period.

The menstrual cycle is made up of four phases. Each phase serves a different function:

  • Menstruation is when you have your period. This is your body shedding your uterine lining from the previous cycle in the absence of pregnancy.
  • The follicular phase, which overlaps with menstruation for the first few days, is when follicles grow. One follicle will generally become larger than the rest and release a mature egg. This signals the end of the follicular phase.
  • Ovulation is when the mature egg is released.
  • The luteal phase begins as the egg travels down the fallopian tube. This phase ends when your next period begins.

Let’s take a closer look at what happens during the luteal phase and what it means if this phase is longer or shorter than normal.

The luteal phase is the second half of your menstrual cycle. It starts after ovulation and ends with the first day of your period.

Once the follicle has released its egg, the egg travels down the fallopian tube, where it may come in contact with sperm and be fertilized. The follicle itself then changes. The empty sac closes off, turns yellow, and transforms into a new structure called the corpus luteum.

The corpus luteum releases progesterone and some estrogen. Progesterone thickens the lining of your uterus so that a fertilized egg can implant. Blood vessels grow inside the lining. These vessels will supply oxygen and nutrients to the developing embryo.

If you get pregnant, your body will also start to produce human gonadotropin (hCG). This hormone maintains the corpus luteum.

HCG enables the corpus luteum to keep producing progesterone until around the 10th week of your pregnancy. Then the placenta takes over progesterone production.

Progesterone levels rise throughout your pregnancy. Here’s a general guide:

  • first trimester: 10 to 44 nanograms per milliliter (ng/mL) of progesterone
  • second trimester: 19 to 82 ng/mL
  • third trimester: 65 to 290 ng/mL

If you don’t get pregnant during this phase, the corpus luteum will shrink and die into a tiny piece of scar tissue. Your progesterone levels will drop. The uterine lining will shed during your period. Then the entire cycle will repeat.

A normal luteal phase can last anywhere from 11 to 17 days. In most women, the luteal phase lasts 12 to 14 days.

Your luteal phase is considered to be short if it lasts less than 10 days. In other words, you have a short luteal phase if you get your period 10 days or less after you ovulate.

A short luteal phase doesn’t give the uterine lining a chance to grow and develop enough to support a growing baby. As a result, it can be harder to get pregnant or it might take you longer to conceive.

A long luteal phase may be due to a hormone imbalance like polycystic ovary syndrome (PCOS). Or, a long lapse since you ovulated could mean that you’re pregnant and you just haven’t realized it yet.

The length of your luteal phase shouldn’t change as you age. But your progesterone levels during this phase may drop as you get closer to menopause.

A short luteal phase can be a sign of a condition called luteal phase defect (LPD). In LPD, the ovary produces less progesterone than usual. Or, the uterine lining doesn’t grow in response to progesterone like it should. LPD can lead to infertility and miscarriage.

Certain lifestyle factors might also be behind a short luteal phase. In one study, women with a short luteal phase were more likely to smoke than those with a longer phase. Smoking might shorten this phase by reducing your body’s estrogen and progesterone production.

To improve your odds of getting pregnant, your doctor can treat LPD with:

  • the infertility drug clomiphene citrate (Serophene) or human menopausal gonadotropins (hMG), which stimulate the growth of follicles
  • hCG to increase progesterone production from the corpus luteum
  • progesterone by mouth, injection, or vaginal suppository

To determine if you’ve ovulated and are in the luteal phase, you can try tracking your basal body temperature (BBT). This is your temperature right when you wake up, before you even get up to use the bathroom or brush your teeth.

During the first part (follicular phase) of your cycle, your BBT will likely hover between 97.0 and 97.5°F. When you ovulate, your BBT will go up because progesterone stimulates heat production in your body.

Once you’re in the luteal phase of your cycle, your basal body temperature should be about 1°F higher than it was during the follicular phase. Look for this temperature bump to tell you that you’ve ovulated and entered the luteal phase.

The luteal phase, which is when the body prepares for pregnancy, can be an important indicator of fertility. If you suspect that you have a long or short luteal phase or that you don’t ovulate, talk to your doctor. They can identify any medical problems affecting your cycle and recommend treatment.

If you’re under 35 and you’ve been trying to get pregnant for at least a year without success, make an appointment with your primary care doctor or a fertility specialist. You could have a fertility problem that’s treatable. Call the doctor after 6 months of trying if you’re 35 or older.