Leutal phase timing

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Here is a complete breakdown of luteal phase timing, drawn from multiple authoritative textbooks:

Luteal Phase Timing

Fig. 10.10 - Costanzo Physiology 7th Edition

When Does It Start?

The luteal phase begins immediately after ovulation. In a standard 28-day cycle, this is day 14. However, the key rule is: ovulation occurs 14 days before the next menses, regardless of total cycle length.

Cycle Length	Ovulation Day	Luteal Phase Starts
24-day cycle	Day 10	Day 10
28-day cycle	Day 14	Day 14
35-day cycle	Day 21	Day 21

Costanzo Physiology, p. 479

How Long Does It Last?

The luteal phase spans days 14-28 in a standard cycle (14 days). Its length is relatively fixed compared to the follicular phase, which is the variable portion of the cycle.

Duration: approximately 12-14 days
The corpus luteum grows for ~7-8 days post-ovulation (peaks at day 21), then begins to involute
It loses secretory function approximately 12 days after ovulation, becoming the corpus albicans
Guyton & Hall Medical Physiology

What Happens During This Phase?

Hormonal sequence:

LH surge triggers ovulation and initiates luteinization of granulosa/theca cells
The corpus luteum forms and secretes progesterone (dominant hormone) and estradiol
Progesterone peaks around ~day 21 (7 days post-ovulation - the "midluteal peak")
Basal body temperature (BBT) rises due to progesterone's thermogenic effect on the hypothalamic set point
FSH and LH are suppressed by estrogen/progesterone feedback and inhibin

Endometrial effect:

The endometrium enters its secretory phase - progesterone stimulates glandular secretion and increases vascularity to prepare for implantation

End of the luteal phase (if no pregnancy):

Without hCG from a fertilized embryo, the corpus luteum regresses
Progesterone and estradiol fall abruptly
This withdrawal triggers endometrial breakdown - menses begins

If pregnancy occurs:

hCG from syncytiotrophoblasts rescues the corpus luteum, maintaining progesterone production until approximately week 9 of gestation when the placenta takes over
Medical Physiology (Boron & Boulpaep), p. 1135

Luteal Phase Defect (LPD)

A shortened or inadequate luteal phase is called luteal phase defect. Diagnostic criteria include:

Low midluteal progesterone level
BBT rise lasting < 11 days
Shortened luteal phase on endometrial biopsy

LPD is associated with infertility and early pregnancy loss, and has been treated with progesterone supplementation (though evidence for routine supplementation remains debated).

Berek & Novak's Gynecology; Tietz Textbook of Laboratory Medicine

Summary Table

Feature	Detail
Phase	Days 14-28 (standard 28-day cycle)
Duration	~14 days (range 12-16 days)
Dominant hormone	Progesterone
Corpus luteum peak	~7-8 days post-ovulation
Corpus luteum involution	~12 days post-ovulation
BBT	Elevated throughout
Ends with	Menses (if no conception)
Rescued by	hCG in pregnancy

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