Time to pregnancy and sex of offspring: cohort study

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.

 
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