We think that one of the best ways to get smart about your fertility is to go right to the research. We had the opportunity to do just that when we asked Dr. Natalie Crawford to explain what any woman or couple looking to conceive should know about her research article titled Prospective evaluation of luteal phase length and natural fertility, that explores how the length of a luteal phase in a woman's menstrual cycle impacts her ability to get pregnant.

FertilitySmarts: Can you give us an overview of the question your research was looking to address?

Dr. Crawford: Our study evaluated if women who had a short luteal phase had a decrease in fecundability (the probability of conceiving in a given menstrual cycle).

FertilitySmarts: Can you explain what a luteal phase is?

Dr. Crawford: The luteal phase is the second half of a woman's cycle. In typical terminology, the start of the menses is considered day 1. The first half of the cycle is known as the follicular phase - the period of time in which an egg develops within a follicle (approximately 2 weeks). After this egg ovulates, the follicle which released the egg forms what is known as the corpus luteum - a cyst that makes progesterone. Progesterone is essential for implantation and survival of early pregnancy. This time period after ovulation and until the next menses is known as the luteal phase.

FertilitySmarts How does the length of a luteal phase impact the chance of getting pregnant?

Dr. Crawford: Progesterone is essential in preparing the endometrium for survival and allowing early pregnancy success prior to progesterone production by the placenta. Excellent and reproducible studies have shown us that removing a woman's corpus luteum (thus ending progesterone production) will end an early pregnancy. Thus, it has long been believed that abnormalities in the luteal phase may decrease a woman's success with pregnancy. The length of the luteal phase represents the survival of the corpus luteal, and clinical signs of a luteal phase deficiency include a shortened luteal phase or spotting during the luteal phase (both of which are considered abnormal).

FertilitySmarts: How is an issue with a luteal phase diagnosed? Do you need a formal diagnosis?

Dr. Crawford: Historically, women were diagnosed with an abnormal luteal phase by performing an endometrial biopsy during the luteal phase. This procedure is painful and can only be performed during a cycle without pregnancy attempt (as it would disrupt an ongoing pregnancy). However, prospective studies failed to show an association between an abnormal biopsy and decreased fertility and are no longer routinely performed. Although there is no standard approach to diagnosing a luteal phase defect (LPD), this does not mean that such a condition does not exist nor does it mean that proper luteal phase function is not important to conception. Currently, carefully evaluating the menstrual cycle characteristics, including the proper charting of the cycle and determination of ovulation, can allow a woman to determine the length of her own luteal phase (defined as the time period from after ovulation to the last day prior to the next menses). In our evolution, we considered a luteal length of 11 days or fewer to be short (with the start of the luteal phase occurring the day after a positive urine ovulation predictor kit test).

FertilitySmarts: What factors can impact luteal phase length?

Dr. Crawford: It is important to realize that clinical symptoms of a luteal phase deficiency are related to either 1) abnormal production of progesterone by the corpus luteum or 2) abnormal endometrial response to normal progesterone production. Thus, factors that may impact either of these processes may predispose a woman to have a short luteal phase.

There are data suggesting that a luteal phase can decrease with age and decreasing ovarian reserve. In addition, in our study, women who were smokers were also found to have a shorter luteal phase. Other studies have shown that vigorous activity (which was not evaluated in our study) may also be associated with a short luteal phase. Most infertility specialists consider a short luteal phase to be a possible marker for abnormal or suboptimal ovulation.

FertilitySmarts: Did your findings change your previous understanding of the luteal phase and its effects?

Dr. Crawford: Our findings expand upon previously held beliefs. Although it is difficult to study the luteal phase (by definition, it does not end in a cycle in which a woman conceives), we evaluated the cycle after one with a short luteal phase. In our study, a short luteal phase decreased short term fecundability (probability of conception), as pregnancy rates for the first 6 months after an isolated cycle with a short luteal phase were decreased. However, we were unable to observe significant differences by 12 months (meaning, not an increase overall in the definition of "infertility").

FertilitySmarts: What is the key takeaway from your research on luteal phase length that anyone trying to conceive should know?

Dr. Crawford: The takeaway message from our research is that the normal luteal phase function is important to conception. Women trying to conceive should consider using fertility awareness methods (FAM) such as charting with basal body temperature (BBT), cervical mucus monitoring, or testing with ovulation predictor tests in order to be aware of their cycle characteristics and seek earlier evaluation by a specialist if abnormalities in the cycle are detected.

It is important to acknowledge that although current tracking apps can help with the collection of cycle data and are easy to use, most tracking apps use a typical calculation of ovulation by presuming a normal luteal phase (subtracting 14 days from the average cycle length). Thus, in women with a short luteal phase, reliance on tracking apps to determine the fertile window is often inaccurate.

Read: How Basal Body Temperature (BBT) Can Help You Get Pregnant

Read: Identifying Stages of Cervical Mucus

Read: Cervical Position and Fertility

Follow Dr. Crawford's fantastic Instagram account @nataliecrawfordmd for fertility-related tidbits and a gimps inside the life of a Reproductive Endocrinologist.