With deep interest, I read the article by Kramer et al on preterm birth syndrome challenges. The authors mention the importance of the way gestational age is determined because differences in menstrual cycle length account for considerable variability in day of conception (between women and usually not in 1 woman). In my opinion they suggest using Ogino-Knaus method (see reference 10 in article) to calculate expected date of delivery in women recalling the first day of the last menstrual period with certainty. Gestational age at any moment in pregnancy is determined by expected date of delivery.

The way in which prevalences of preterm, term, and postterm births are influenced by gestational age calculation, either using amenorrhea or corrected for differences in follicular phase length, is graphically visualized by Boyce et al. They used, like Saito et al (reference 9 in article), temperature curves to determine ovulation.

The relation between follicular phase length and date of delivery is also brought forward by Carus. He described due date calculation in 3 situations that are follicular phase length related. Firstly, multiparae (in that era) often correctly calculated due date since they knew the conception date. Secondly, when conception date was unknown, the physician deduced an accurate estimate; due date was 280 days later. Carus noticed that in this group, women often gave birth earlier or later than due date and wrote that when the last menstrual period occurred 8-14 days before conception, delivery also started ≥8 days earlier than expected. Based on experience, he questioned the accuracy of 280 days and stated that conception did not always take place 280 days before delivery. Thirdly, when conception date could not be estimated accurately, the first day of the last menstrual period was used. Due date was then 40-42 weeks later because conception might have occurred 8-14 or even 21 days after the first day. The due date of Carus matches 42 ^0/7 weeks of gestation nowadays; accepting that pregnancy duration is 280 days (including 14 days without embryo) from the first day of the last normal menstrual period in women with regular menstrual cycles of 28 days.

Carus could be credited for relating differences in preovulatory phase length to due date calculation and Ogino and Knaus for describing the constant postovulatory phase. As regards studies on preterm birth syndrome, one wonders whether heterogeneity decreases if only naturally conceived pregnancies with known ovulation are included.