妊娠確率と性交のタイミングに関する予備的解析
A preliminary analysis on fecundability and timing of sexual intercourse
小西祥子（東京大学/ワシントン大学）、早乙女智子（京都大学）、清水慶子（岡山理科大学）、大庭真梨（東邦大学）、鶴巻香奈子（東京医療保健大学）
Shoko Konishi (The University of Tokyo, University of Washington),
Tomoko Saotome (Kyoto University), Keiko Shimizu (Okayama University of Science),
Mari S. Oba (Toho University), Kanako Tsurumaki (Tokyo Healthcare University),

moe@humeco.m.u-tokyo.ac.jp（小西祥子）
Background: Probability of conception per menstrual cycle (fecundability) is linked to incidenceof infertility. Considering that compared to Western couples Japanese couples tend to show
lower frequency of intercourse and that fertile period in a menstrual cycle lasts only for a weekor so, timing of intercourse might matter more for Japanese couples as to a woman can achievea pregnancy or not in a specific menstrual cycle.
Objectives: To analyze how frequency and timing of sexual intercourse affects fecundability.
Methods: A prospective cohort study was conducted in Japan targeting nulliparous women(n=80) who were trying to conceive and had no known history of infertility. The participants
were followed up until confirmation of clinical pregnancy, or for up to 24 weeks, during whichdaily information on menstrual bleed and sexual intercourse (with or without insemination) wascollected. The participants also tested ovulation and pregnancy using home test kits. For
statistical analysis first day of positive ovulation test in each cycle was labelled F-1. In eachcycle eight days, i.e. F-6 (5 days before F-1) to F1 (2 days after F-1), were defined to cover thefertile period. Baseline characteristics were compared between participants who did vs. did notconceive during follow-up. Using the cycle-level data logistic regression analysis wasconducted with conception in each cycle as an outcome and frequency and timing of intercourse
as explanatory variables.
Results: During follow-up 35 out of 80 women conceived. The mean (SD) number of days withintercourse with insemination was 3.2 (3.0) per menstrual cycle and it was 0.4 (1.4) forintercourse without insemination (n=367 menstrual cycles). Women who discontinued
contraception 24+ months before the survey were less likely to conceive (Table 1). The multiple
logistic regression analysis revealed that intercourse on F-1 is positively associated withfecundability, while number of days with intercourse per cycle was not (Table 2). Unexpectedly
we found that probability of conception was higher if a cycle had sexual intercourse without
insemination, after adjusting for frequency and timing of intercourse with insemination.

Table 1 Basic characteristics of the participants at study baseline by conception duringfollow-up (n=80). Mean (SD) or n (%). P-values by Wilcoxon’s rank sum test for continuous
variables and by Fisher’s exact test for categorical variables.
Conceived during follow-up
Characteristics
Age (year)
Partner’s age (year)

Total (n=80)

No (n=45)

Yes (n=35)

P-value29 (3)

30 (3)

29 (3)

0.099

32 (5)

32 (5)

32 (4)

0.981

2BMI (kg/m )

0.011

<18.5

12 (15%)2 (4%)

10 (29%)18.5-22.9

56 (70%)36 (80%)20 (57%)23.0+

12 (15%)7 (16%)

5 (14%)

Smoker4 (5%)

2 (4%)

2 (6%)

1.000

Duration since discontinuing
contraception (month)

0.004

0-1150 (63%)

25 (56%)25 (71%)

12-23

14 (18%)5 (11%)

9 (26%)

24+

11 (14%)10 (22%)

1 (3%)Do not remember

5 (6%)

5 (11%)

0 (0%)

Table 2. Logistic regression analysis on conception and frequency and timing of sexualintercourse (n=367 menstrual cycles; n=35 with and n=332 without conception).
Odds ratio (95% confidence interval)
Explanatory variable

Bivariate models

Multivariate modelc

Intercoursea on F-6

1.5 (0.5, 3.5)

0.9 (0.3, 2.4)

Intercoursea on F-5

2.8 (1.2, 6.0)

2.3 (0.9, 5.8)

Intercourse on F-4

1.9 (0.8, 4.1)

1.7 (0.6, 4.5)Intercoursea on F-3

1.4 (0.5, 3.2)

1.1 (0.4, 2.6)

Intercourse on F-2

1.1 (0.5, 2.5)

1.0 (0.3, 2.4)

Intercoursea on F-1

3.8 (1.9, 8.1)

3.5 (1.6, 7.6)

1.0 (0.5, 2.2)

0.8 (0.3, 1.8)

1.2 (0.4, 2.6)

0.8 (0.3, 2.1)

Number of days with intercourse per cycle

1.1 (1.0, 1.3)

1.1 (0.9, 1.2)

Any days with intercourseb per cycle

2.9 (1.4, 5.9)

2.3 (1.1, 4.9)

aaaIntercourse on F0Intercoursea on F1aa: with insemination, b: without insemination, c: all the explanatory variables shown in the tablewere included in the model