What Is the Luteal Phase?

Your cycle has two halves. The first, the follicular phase, is the time it takes an egg to mature, and it is the part that stretches or shrinks when your cycle length changes. The second half, the luteal phase, starts once the egg is released. It is shorter, steadier and the part where an embryo implants, which is why people trying to conceive pay close attention to it.

What is the luteal phase?

The luteal phase is the stretch of your cycle from just after ovulation to the day before your next period starts. After the follicle releases its egg, what remains becomes the corpus luteum, a temporary gland that secretes progesterone.¹ Progesterone readies the uterine lining for an embryo, and in natural cycles it is essential to establishing and maintaining a pregnancy until the placenta takes over.¹

If no pregnancy implants, the corpus luteum winds down, progesterone falls and your period starts. If a pregnancy does implant, rising hCG from the pregnancy keeps the corpus luteum making progesterone; if hCG fails to rise, the corpus luteum fails and progesterone falls.¹

How long is the luteal phase?

ASRM describes a typical luteal phase as relatively fixed at 12 to 14 days, ranging from 11 to 17 days.¹ Different studies land in that band depending on how they measured ovulation:

In the large app dataset, average luteal length rose from 8.0 days in very short cycles (15 to 20 days) to 12.9 days in long cycles (36 to 50 days), but differences in cycle length came mostly from the follicular phase.³ That is why calculators assume a fixed second half, and why you get a better estimate by entering your own luteal length in the ovulation calculator.

What happens during the luteal phase

Progesterone peaks in non-pregnant cycles about 6 to 8 days after ovulation.¹ That is also when an embryo arrives and implants. In a study of 221 women trying to conceive, pregnancies that lasted six weeks or more implanted 6 to 12 days after ovulation, and 84% implanted on day 8, 9 or 10.⁴ The DPO calculator shows where you are in that timeline and when a test can give a reliable answer.

How to work out your luteal phase length

Count from the day after ovulation to the day before your next period. You need a reasonable estimate of ovulation day, which you can get two ways:

• LH tests. Ovulation usually follows the start of the urinary LH surge by about a day.⁹ The day after your first positive test is a fair estimate. See how long after an LH surge you ovulate.
• Temperature. Track your basal body temperature and take the last low reading before the sustained rise as your estimated ovulation day, as the large app study did.³

Track at least two or three cycles. A single short or long cycle tells you very little.

BBT after ovulation

After ovulation, basal body temperature rises by about 0.5 to 1°F and stays higher until the end of the cycle.⁵ One common charting rule draws a coverline and looks for three temperatures in a row that are at least 0.2°F above the highest of the previous six.⁶ Because the rise comes after the egg is released, ACOG notes that temperature charting shows when ovulation has happened, not when it is coming.⁵

Temperature is easy to throw off with fever, alcohol, stress or broken sleep.⁶ Some devices take readings overnight instead of once on waking. Natural Cycles is FDA-cleared as a contraceptive app and also offers a plan-pregnancy mode, and it uses temperature to estimate ovulation.

A rise in urine PdG, a progesterone breakdown product, is another way to confirm ovulation. In 40 women with ultrasound-confirmed ovulation, PdG rose after ovulation in every case and peaked a median of 7.5 days later.⁹ Some home monitors read PdG alongside other hormones.

What is luteal phase defect, and is it real?

Luteal phase deficiency (LPD), also called luteal phase defect, is the idea that a luteal phase that is too short, or too low in progesterone, makes implantation fail. ASRM's current committee opinion, updated in 2026, describes it as a clinical diagnosis that may be present with a luteal phase of 10 days or less.¹ It also states that LPD "has not been proven to be an independent entity causing infertility or recurrent pregnancy loss."¹

The controversy has three parts.

1. Short phases are common in fertile women. In the BioCycle Study of regularly menstruating women, 8.9% of ovulatory cycles had a luteal phase under 10 days.⁸ In the large app dataset, 18% of cycles had a luteal phase under 11 days.³
2. The effect on fertility is small and temporary. In a study of 284 women trying to conceive, a short luteal phase (11 days or less, counting ovulation day) occurred in 18% of cycles. Women had 0.82 times the odds of pregnancy in the following cycle, but cumulative pregnancy rates at 12 months were not significantly different.⁷
3. No reliable test or treatment exists. ASRM concludes that no diagnostic test for LPD has been proven reliable in practice or able to tell fertile from infertile women, and that endometrial biopsy is not recommended for this purpose.¹ It also states that no treatment for LPD has been shown to improve pregnancy rates in natural, unstimulated cycles.¹

Abnormal luteal function can result from other conditions, including thyroid disease, high prolactin, hypothalamic amenorrhea, eating disorders, excessive exercise and PCOS.¹ ASRM's advice is to look for and treat those causes when they are present.¹

When to talk to a clinician

Bring it up if your luteal phase is repeatedly 10 days or shorter, or if your cycles are irregular or your periods are absent. Take your charts or test results with you. For the rest of this step, including timing and testing guides, see the Try hub.