The Mystery and Importance of the Luteal Phase

Quick definition: The portion of your menstrual cycle that occurs after ovulation but before the first day of your next menstrual cycle is called the luteal phase. On average, the luteal phase lasts between 10 to 14 days.

Some women with fertility problems have a short luteal phase. Recurrent miscarriage—miscarrying two or more times in a row—is also associated with a shorter than normal luteal phase.

Problems during the luteal phase are sometimes referred to as a luteal phase defect. However, some women with normal fertility have a short luteal phase. The connection between luteal phase length and fertility is unclear. More on this below.

The menstrual cycle can be broken into two main parts: the follicular phase and the luteal phase.

The follicular phase is all about ovulation. Hormones trigger changes in ovarian follicles until, eventually, a mature egg is ovulated. Follicles are small sacs in which the eggs of your ovaries develop. The scientific name for egg development and maturation is oogenesis.

The luteal phase is all about preparing the endometrium and body for pregnancy. Your body is very optimistic and assumes the ovulated egg was fertilized. After ovulation, the follicle that released the egg becomes a corpus luteum. The corpus luteum secretes estrogen and progesterone.

While estrogen is important, progesterone may be the single most important hormone during the luteal phase. Progesterone has many roles, including...

• Suppressing the hormones GnRH, FSH, and LH: hormones that stimulate the ovaries and cause ovulation. Otherwise, you might conceive again after you already were pregnant.
• Preparing the endometrium: progesterone triggers the uterine lining (or endometrium) to secrete special proteins, which will nourish an embryo
• Prevent menstruation: progesterone stops the endometrium from breaking down, which could lead to miscarriage if you’re pregnant

Progesterone also causes your body temperature to rise. If you chart your basal body temperature, you’ll notice a slight rise in temperature after ovulation. If you get pregnant, your temperature will stay elevated beyond your regular luteal phase length. If you don’t get pregnant, your temperature will start to drop just before your period arrives. The drop in progesterone levels both lowers your body temperature and starts menstruation.

Progesterone is also responsible for luteal phase symptoms—something many women confuse for early pregnancy signs.

On average, the luteal phase is between 12 and 14 days. However, it can be as short as 8 days and as long as 16 days. Whatever your regular luteal phase length is, it’ll tend to be consistently that length every cycle.

So, for example, a woman whose luteal phase tends to be 12 days will always be 11 to 13 days long. If her luteal phase goes longer than 13 days, that may be an early sign of pregnancy.

A luteal phase shorter than 8 (or 10) days may indicate a potential fertility problem. But not necessarily. While women who struggle to conceive or experience repeated miscarriage may tend to have shorter luteal phases, it’s possible for a woman with good fertility to have a short luteal phase.

You may learn what your luteal phase length is from basal body temperature charting. If you’re charting, and you notice a short luteal phase, don’t worry yet. As long as you don’t have any other symptoms of infertility, it could be normal for you.

However, if you don’t get pregnant after one year of trying to conceive (or after six months, if you’re 35 years of age or older), be sure to see your doctor. You should also see your doctor if you notice other worrisome symptoms.

A luteal phase defect is a theoretical cause of infertility and miscarriage. It is defined as having low or inadequate levels of progesterone during the luteal phase. However, it is considered a theoretical cause of infertility and early miscarriage. There’s a lot of debate and controversy surrounding this diagnosis.

The American Society for Reproductive Medicine's position on luteal phase defects  is that the condition does not exist as a standalone cause of infertility.

The controversy is due to…

• Diagnosis difficulties (Research hasn’t found a definitive way to test or confirm a luteal phase defect.)
• Unclear outcomes (A short luteal phase or low progesterone level doesn’t always cause infertility or repeat miscarriage.)
• Uncertain treatment outcomes (It’s unclear if proposed treatments really improve fertility or can prevent miscarriage.)

Possible symptoms or problems that are associated with a luteal phase defect include:

• Spotting between ovulation and your expected period
• Repeated early miscarriage
• Irregular ovulation or anovulation
• Short menstrual cycles

Possible causes of a luteal phase defect include:

• A thyroid imbalance
• Excessive exercise
• Eating disorders, like anorexia
• Age (being over age 35)
• Obesity
• Polycystic ovarian syndrome (PCOS)
• Endometriosis
• Hyperprolactinemia
• IVF treatment that includes the use of GnRH agonists

As mentioned above, there’s no research-based way to diagnose a luteal phase defect. Every method has possible problems. That said, here are some ways a luteal phase defect may be determined:

Basal body temperature (BBT) charting: Charting can show an abnormally short luteal phase. However, research has found that the exact day of ovulation isn’t always accurately indicated on a BBT chart. This means it’s not certain how many days the luteal phase is. It may be longer (or short) than the chart indicates.

Progesterone level testing: Progesterone levels may be tested six to eight days after ovulation occurs. However, what progesterone levels should be considered normal is unclear.

Another potential problem is timing. While progesterone levels peak about a week after ovulation, knowing exactly which day ovulation occurred isn’t simple. That means knowing when to test isn’t clear either.

Endometrial biopsy: Endometrial biopsy involves looking at endometrial tissue during the luteal phase, and evaluating whether the cells look like they are in the correct stage of growth (in relation to where a woman is in her menstrual cycle.)

This was once considered the gold standard for diagnosis of a luteal phase defect. However, studies have found inconsistent results. Results that were considered abnormal didn’t necessarily lead to poor pregnancy results.

Can a luteal phase defect cause early miscarriage? And can treating a luteal phase defect with progesterone solve this problem? This is a complicated question, one that is frequently given the quick answer of, “Since luteal phase defects may not exist, the answer is no.” However, the real answer is more complicated than that.

There is not sufficient evidence to say that a luteal phase defect (defined as a short luteal phase or low progesterone) alone will cause miscarriage or infertility. For women who have not had repeated miscarriages, studies have found that progesterone supplementation does not reduce the general risk of miscarriage in women.

That said, recent studies have found that women who have experienced repeat miscarriages (two or more miscarriages in a row) may benefit from progesterone supplementation. This seems to be more likely if the woman has experienced three or more miscarriages in a row. 

As mentioned above, an effective treatment for luteal phase defect isn’t clear.

That said, your doctor may consider any of the following:

• Treating underlying conditions first: If there is a thyroid imbalance, for example, treat that first.
• Boost ovulation with fertility drugs: By boosting ovulation, the corpus luteum may be stronger, leading to a healthier luteal phase.
• Progesterone supplementation: It's highly controversial whether progesterone supplements can improve fertility or reduce early miscarriage. As mentioned above, progesterone may be beneficial for women who have had three or more miscarriages.
• hCG injections: Low doses of hCG (an injectable fertility drug) may be given through the luteal phase. However, progesterone supplementation is more common, due to having fewer side effects.

With IVF treatment, progesterone supplementation has been shown to improve the luteal phase and improve pregnancy outcomes. Progesterone injections or progesterone suppositories may be prescribed.