Defining Term Maturation
The duration of pregnancy is the single, most important benchmark of the fetus’ maturity. The preferred duration of pregnancy, 39 weeks, is called – term pregnancy. We mirror this naming approach for oocyte maturation by postulating the existence of the term oocyte maturation, representing the number of days required to produce a viable (high-quality) oocyte, defined as being able to develop to term once fertilized. The term maturation is calculated from the first day of the period to the beginning of the gonadotropins surge and is estimated at about 16.5 days.
The menstrual cycle is divided into the follicular phase (from the first day of the period to ovulation) and the luteal phase (the days after ovulation and before the next period). It is well known and not controversial that the duration of the luteal phase is highly conserved and varies very little from female to female and does not change with age, while the duration of the follicular phase varies from woman to woman and does change with age. This is why when we see changes in the duration of the menstrual cycle, we can safely assume that it is solely due to the follicular phase changes. There are exceptions, such as PCOS, but this section is limited to discussing the rule. Term maturation is the state of the oocyte when it acquires the ability to develop to term after fertilization. It is counted from the first day of the period to the beginning of the gonadotropins surge (or administration of ovulation trigger). In order to determine the duration of term maturation, first, we need to know the duration of the follicular phase that would most likely result in pregnancy. According to Bull et al (Nature, 2019), this comprises 18 days.
Further, it is well established that intrafollicular oocyte maturation is terminated within minutes of the gonadotropins surge, which begins about 37 hrs (1.5 days) before ovulation. Therefore to determine term maturation, we need to subtract those 1.5 days from 18 days = 16.5 days. Thus, in the first approximation, term maturation is 16.5 days.
Based on clinical observations, 10 days of controlled follicular phase during IVF cycle (about 8 days of stimulation) represents the time-cliff, when many patients will not be able to produce competent eggs.
Importantly, unlike the term pregnancy, which is the same for any woman, we believe that term maturation has considerable variability from woman to woman. Also, with age, it may take longer for an oocyte to reach term maturation.
While not truly scientific, the graph below helps to understand a comparison of the impact of a loss in the span of a few days during the follicular phase. In the graph below the duration of the follicular phase is plotted in proportion to the duration of pregnancy.