It is an old idea that fecundity may correlate with the sex ratio and many of the theories related to frequency of males to females (sex ratio) would imply such an association. In Smits et al's paper,1 they put forward the idea that cervical mucous viscosity correlate with both the penetration of Y bearing spermatozoa and fecundity. They find a striking empirical support for the hypothesis using a Dutch data source with almost 5300 births.
We expect that many have looked into this hypothesis but found no association and did not or could not publish their negative results.
They refer to our previous negative study2 and suggest that our data were confounded by infertility treatment. We therefore report a replication of their study using the Danish National Birth Cohort that has data on infertility treatment and time to pregnancy. The cohort includes data from 100,000 pregnancies and has been described in detail elsewhere.3 In brief, the data on infertility treatment and time to pregnancy were reported early in pregnancy, and the data on time to pregnancy were used the following categories: 0, 1-2, 3-5, 6-12, and >12 months. Data on the sex of the newborns came from the National Birth Register. We identified all planned first pregnancies for each woman in the cohort. In total, 61249 singletons and 3156 twins were available for analysis. Our replication of Smits et al's calculations is shown in the table.
Table Percentages of male infants and odds ratios of being male gender according to time to pregnancy and infertility treatment
Time to pregnancy or infertility treatment No. of births Male infants Crude OR Adjusted OR (95% CI)
n %
Among singletons
0 months 13639 7024 51.5 1.00 reference
1-2 months 14400 7425 51.6 1.00 1.00 (0.96-1.05)
3-5 months 13096 6657 50.8 0.97 0.97 (0.93-1.02)
6-12 months 9762 4950 50.7 0.97 0.97 (0.92-1.02)
>12 months 5764 2943 51.1 0.98 0.98 (0.92-1.04)
With infertility treatment 4588 2362 51.5 1.00 0.99 (0.92-1.06)
Among twins
0 months 415 220 53.0 1.00 reference
1-2 months 396 214 54.0 1.04 1.04 (0.78-1.37)
3-5 months 362 187 51.7 0.95 0.91 (0.68-1.21)
6-12 months 193 106 54.9 1.08 1.04 (0.74-1.48)
>12 months 100 48 48.0 0.82 0.77 (0.49-1.20)
With infertility treatment 1690 873 51.7 0.95 0.87 (0.69-1.11)
Logistic regression;
OR: odds ratio;
CI: confidence interval;
Adjusted for maternal age, parity, pre-pregnancy body mass
index, smoking, alcohol use, month of conception, and
regularity of the menstrual cycle.
We found no association between time to pregnancy and sex ratio in singletons or in twins. Infertility treatment was also not associated with the sex ratio and is an unlikely explanation for our previous results.
Jørn Olsen, MD, PhD, Professor
The Danish Epidemiology Science Centre, University of Aarhus, Vennelyst Boulevard 6, DK 8000 Aarhus C, Denmark; and Department of Epidemiology, School of Public Health, UCLA, Los Angeles, USA. Email: jo@ucla.edu or jo@soci.au.dk.
Weijin Zhou, MD, PhD
Shanghai Institute of Planned Parenthood Research, 20032 Shanghai, China. Email: zw0822@yahoo.com.
Jin Liang Zhu, MD, PhD
The Danish Epidemiology Science Centre, University of Aarhus, Vennelyst Boulevard 6, DK 8000 Aarhus C, Denmark. Email: zjl@soci.au.dk.
References
1. Smits LJ, de Bie RA, Essed GG, van den Brandt PA. Time to pregnancy and sex of offspring: cohort study. BMJ 2005;331:1437-1438.
2. Weijin Z, Olsen J. Offspring sex ratio as an indicator of reproductive hazards. Occup Environ Med 1996;53:503-504.
3. Olsen J, Melbye M, Olsen SF, Sorensen TI, Aaby P, Andersen AM, et al. The Danish National Birth Cohort - its background, structure and aim. Scand J Public Health 2001;29:300-307.
Competing interests:
None declared
Competing interests:
Table Percentages of male infants and odds ratios of being male gender according to time to pregnancy and infertility treatmentTime to pregnancy or infertility treatment No. of births Male infants Crude OR Adjusted OR (95% CI) n % Among singletons 0 months 13639 7024 51.5 1.00 reference1-2 months 14400 7425 51.6 1.00 1.00 (0.96-1.05)3-5 months 13096 6657 50.8 0.97 0.97 (0.93-1.02)6-12 months 9762 4950 50.7 0.97 0.97 (0.92-1.02)>12 months 5764 2943 51.1 0.98 0.98 (0.92-1.04)With infertility treatment 4588 2362 51.5 1.00 0.99 (0.92-1.06)Among twins 0 months 415 220 53.0 1.00 reference1-2 months 396 214 54.0 1.04 1.04 (0.78-1.37)3-5 months 362 187 51.7 0.95 0.91 (0.68-1.21)6-12 months 193 106 54.9 1.08 1.04 (0.74-1.48)>12 months 100 48 48.0 0.82 0.77 (0.49-1.20)With infertility treatment 1690 873 51.7 0.95 0.87 (0.69-1.11)Logistic regression;OR: odds ratio;CI: confidence interval;Adjusted for maternal age, parity, pre-pregnancy body mass index, smoking, alcohol use, month of conception, and regularity of the menstrual cycle.
21 December 2005
Jørn Olsen
Professor
Weijin Zhou, Jin Liang Zhu
Department of Epidemiology, School of Public Health, UCLA, Los Angeles, USA
Rapid Response:
Time to pregnancy and sex of offspring
Dear Sir,
It is an old idea that fecundity may correlate with the sex ratio and many of the theories related to frequency of males to females (sex ratio) would imply such an association. In Smits et al's paper,1 they put forward the idea that cervical mucous viscosity correlate with both the penetration of Y bearing spermatozoa and fecundity. They find a striking empirical support for the hypothesis using a Dutch data source with almost 5300 births.
We expect that many have looked into this hypothesis but found no association and did not or could not publish their negative results.
They refer to our previous negative study2 and suggest that our data were confounded by infertility treatment. We therefore report a replication of their study using the Danish National Birth Cohort that has data on infertility treatment and time to pregnancy. The cohort includes data from 100,000 pregnancies and has been described in detail elsewhere.3 In brief, the data on infertility treatment and time to pregnancy were reported early in pregnancy, and the data on time to pregnancy were used the following categories: 0, 1-2, 3-5, 6-12, and >12 months. Data on the sex of the newborns came from the National Birth Register. We identified all planned first pregnancies for each woman in the cohort. In total, 61249 singletons and 3156 twins were available for analysis. Our replication of Smits et al's calculations is shown in the table.
We found no association between time to pregnancy and sex ratio in singletons or in twins. Infertility treatment was also not associated with the sex ratio and is an unlikely explanation for our previous results.
Jørn Olsen, MD, PhD, Professor
The Danish Epidemiology Science Centre, University of Aarhus, Vennelyst Boulevard 6, DK 8000 Aarhus C, Denmark; and Department of Epidemiology, School of Public Health, UCLA, Los Angeles, USA. Email: jo@ucla.edu or jo@soci.au.dk.
Weijin Zhou, MD, PhD
Shanghai Institute of Planned Parenthood Research, 20032 Shanghai, China. Email: zw0822@yahoo.com.
Jin Liang Zhu, MD, PhD
The Danish Epidemiology Science Centre, University of Aarhus, Vennelyst Boulevard 6, DK 8000 Aarhus C, Denmark. Email: zjl@soci.au.dk.
References
1. Smits LJ, de Bie RA, Essed GG, van den Brandt PA. Time to pregnancy and sex of offspring: cohort study. BMJ 2005;331:1437-1438.
2. Weijin Z, Olsen J. Offspring sex ratio as an indicator of reproductive hazards. Occup Environ Med 1996;53:503-504.
3. Olsen J, Melbye M, Olsen SF, Sorensen TI, Aaby P, Andersen AM, et al. The Danish National Birth Cohort - its background, structure and aim. Scand J Public Health 2001;29:300-307.
Competing interests:
None declared
Competing interests: Table Percentages of male infants and odds ratios of being male gender according to time to pregnancy and infertility treatmentTime to pregnancy or infertility treatment No. of births Male infants Crude OR Adjusted OR (95% CI) n % Among singletons 0 months 13639 7024 51.5 1.00 reference1-2 months 14400 7425 51.6 1.00 1.00 (0.96-1.05)3-5 months 13096 6657 50.8 0.97 0.97 (0.93-1.02)6-12 months 9762 4950 50.7 0.97 0.97 (0.92-1.02)>12 months 5764 2943 51.1 0.98 0.98 (0.92-1.04)With infertility treatment 4588 2362 51.5 1.00 0.99 (0.92-1.06)Among twins 0 months 415 220 53.0 1.00 reference1-2 months 396 214 54.0 1.04 1.04 (0.78-1.37)3-5 months 362 187 51.7 0.95 0.91 (0.68-1.21)6-12 months 193 106 54.9 1.08 1.04 (0.74-1.48)>12 months 100 48 48.0 0.82 0.77 (0.49-1.20)With infertility treatment 1690 873 51.7 0.95 0.87 (0.69-1.11)Logistic regression;OR: odds ratio;CI: confidence interval;Adjusted for maternal age, parity, pre-pregnancy body mass index, smoking, alcohol use, month of conception, and regularity of the menstrual cycle.