Short Luteal Phase

A short luteal phase has long been attributed as a cause of infertility and miscarriage. We dig deep into the research and look at how a luteal phase defect can influence reproduction.

What is the luteal phase

The menstrual cycle can be divided in two. During the first part of the cycle, called the follicular phase, the ovary develops eggs inside follicles which begin to grow. After several days one follicle grows larger than the others. This dominant follicle then goes on to release an egg at ovulation.

The dominant follicle’s job does not finish with ovulation. Now called a corpus luteum, the follicle produces progesterone for the rest of the cycle. Hence the second half of the cycle is known as the luteal phase.

If you look in a textbook, each half of the cycle is an equal 14 days in length resulting in a 28 day cycle. However, women aren’t like textbooks. Cycles can vary in length and studies looking at when ovualtion occurs show that this can also vary from cycle to cycle, even in women with regular periods.

Progesterone is the hormone responsible for maintaining the lining of the womb and is essential for pregnancy. The name itself literally means prenancy hormone, derived from the latin pro-, meaning “for”, gest-, referring to pregnancy (as in “gestation”) and -one, as in hormone. Therefore, it doesn’t take a genious to think that fertility problems or early pregnancy loss is due to a deficiency in progesterone. In fact, the first scientific reference to a “luteal phase defect” was published in 1949.¹

A supposed luteal phase defect has subsequently been suggested to be responsible for the following⁵:

• infertility
• first-trimester miscarriage
• short cycles
• premenstrual spotting

And possible causes:

• anorexia, starvation, and eating disorders
• excessive exercise
• stress
• obesity and polycystic ovary syndrome (PCOS)
• endometriosis
• aging
• inadequately treated 21-hydroxylase deficiency
• thyroid dysfunction and hyperprolactinemia
• ovulation stimulation
• ovulation induction with or without gonadotropin-releasing agonists
• assisted reproductive technology
• during the postpartum period, with significant weight loss or exercise.

… and in random cycles of normally menstruating women.

So far, the story sounds good, but digging a little deeper things start to fall apart. Science and medicine need more than just a good story and should be backed up with research to answer the following questions.

1. How do you define luteal phase defect?
2. Is luteal phase defect associated with infertility and pregnancy loss?
3. Does treatment for a luteal phase defect improve reproductive outcomes?

Unfortunately, the research falls short when trying to address these questions.

How short is a piece of string? Defining a short luteal phase.

The “short luteal phase” was initially described as an interval of less than or equal to 8 days from the luteinizing hormone (LH) peak to the onset of menstrual flow. Alternative definitions have also been used, including a luteal phase of less than or equal to 10 days in length, but also 11 and 9 days. Definitions based on blood progesterone levels have also been proposed. Conflicting definitions make it difficult to understand exactly who has a short luteal phase.

What causes a short luteal phase?

Given that the definition is vague, trying to explain why some women have a short luteal phase is difficult. Proposed explanations include problems with the first part of the cycle, the follicular phase, or abnormal hormone control of the menstrual cycle from the brain. Disorders of thyroid and prolactin have also been implicated along with kidney problems, breast-feeding, obesity and age.

However, perhaps the most important association is that a short luteal phase may occur in healthy young women.²

Perhaps the most important association is that a short luteal phase may occur in healthy young women.²

Diagnosis

Tracking the exact time of ovulation is notoriously difficult. Different methods for diagnosing a short luteal phase are all based on identifying when ovulation occurred. The graphic summarises some of these methods and their pros and cons.

Method	Advantages	Disadvantages
Tracking basal body temperature (BBT)	Just need a thermometer	– inaccurate – inconvenient
Urinary ovulation predictor kits that detect the LH surge	Easy to perform and readily available	– not always accurate – can cause anxiety – a short luteal phase may occur in healthy women -a false-positive LH surge is found when testing urine in >7% of cycles in women with regular menstrual cycles.3
Progesterone blood levels	Provides evidence of ovulation, but not not when	– Progesterone is secreted in pulses and levels can vary up to 8 times in 90 minutes.4 – Unfortunately, there is no recognised pattern of progesterone levels during the luteal phase in normal fertile women. – No minimum progesterone level defines “fertile” luteal function.

Problems with progesterone measurement

The corpus luteum varies from cycle to cycle in normal fertile women. Therefore, random serum progesterone levels are not valid between cycles. Furthermore, progesterone levels can fluctuate throughout the day. The highest level can be as much as 12 times higher than the lowest level, so a “low” progesterone doesn’t necessarily reflect the true picture.

Once pregnancy has been established the corpus luteum is stimulated by the pregnancy hormone, hCG to produce progesterone. Progesterone levels have some value in predicting if the pregnancy is nonviable or extrauterine, but again this is limited.

Low progesterone levels in early pregnancy are likely reflect abnormal hCG stimulation of the corpus luteum by a nonviable or extrauterine pregnancy rather than a problem with the corpus luteum. A low progesterone level obtained at the time of, or after, diagnosis of early pregnancy should not be used to initiate therapy with exogenous progesterone.

Is luteal phase defect associated with infertility and pregnancy loss?

Ultimately, the main aim of trying to conceive is getting pregnant. I can understand the appeal of the concept of a short luteal phase to explain why pregnancy is not happening. Nevertheless, when comparing women with a short luteal phase to those who do not, the rate of infertility at a year is not significantly higher.⁶

Does treatment for a luteal phase defect improve reproductive outcomes?

Given that the definition of luteal phase defect is variable and results from studies include both fertile and infertile women the research looking at potential treatments is not very good quality.

If a woman has another underlying problem such as hypothalamic amennorrhoea, high prolactin or obesity, these problems should be treated. Nevertheless, beyond that the only potential treatment is progesterone supplementation.

Progesterone can be given as tablets, injections or pessaries. Commonly, progesterone is used as part of assisted conception treatments when the natural cycle has been modified with drugs which impair progesterone production.

Currently, there is no evidence that progesterone supplementation provides any benefit for women having natural cycles. No randomised-controlled trials have been done to investigate progesterone supplementation for women with a luteal phase defect.

Take home messages

• A luteal phase defect has no clear definition and has not been proven to cause infertility or pregnancy loss.
• A short luteal phase can be found in normal healthy women.
• Women with a “short luteal phase” have the same pregnancy outcomes over a year compared with those with “normal” cycles.

Sources:

1. Jones G. Some newer aspects of the management of infertility. Journal of the American Medical Association. 1949.
2. Strott T. The short luteal phase. The Journal of Clinical Endocrinology & Metabolism. 1970.
3. McGovern P. Absence of secretory endometrium after false-positive home urine luteinizing hormone testing. Fertility and Sterility. 2004.
4. Filicori M. Neuroendocrine regulation of the corpus luteum in the human. Evidence for pulsatile progesterone secretion.. The Journal of Clinical Investigation. 1984.
5. ASRM. Current clinical irrelevance of luteal phase deficiency: a committee opinion. Fertility and Sterility. 2015.
6. Crawford N. Prospective evaluation of luteal phase length and natural fertility. Fertility and Sterility. 2017.