Table 1.
Mothers' and infants' characteristics in the three cohorts
| IVF-ICSI twins | Control twins | P value* | IVF-ICSI singletons | P value† | |
|---|---|---|---|---|---|
| No of babies born alive | 3393 | 10 239 | 5130 | ||
| No of stillborn babies‡ | 45 (13.1/1000) | 123 (11.8/1000) | 0.6 | 34 (6.6/1000) | 0.002 |
| Mean children's age at follow up in years (SD) | 4.2 (1.7) | 4.4 (1.7) | <0.001 | 4.1 (1.7) | 0.12 |
| Sex of babies:§ | |||||
| No (%) of boys | 1747/3352 (52.1) | 5237/10 215 (51.3) | 0.4 | 2700 (52.6) | 0.6 |
| No (%) of girls | 1605/3352 (47.9) | 4978/10 215 (48.7) | 2430 (47.4) | ||
| No (%) of twin pairs of the same sex | 851/1676 (50.8) | 3330/5103 (65.4) | <0.001 | — | |
| Mean birth weight in g (SD) | 2508 (615) | 2540 (612) | 0.01 | 3457 (629) | <0.001 |
| Mean gestational age in weeks (SD) | 35.9 (3.0) | 36.1 (2.9) | 0.02 | 39.3 (2.2) | <0.001 |
| No of deaths per 1000 babies¶ | |||||
| Early neonatal, up to day 6 | 26 (7.6) | 119 (11.6) | 0.05 | 23 (4.5) | 0.06 |
| Late neonatal, day 7-28 | 4 (1.2) | 22 (2.1) | 0.3 | 3 (0.6) | 0.4 |
| Day 29-1 year | 5 (1.5) | 13 (1.3) | 0.8 | 11 (2.1) | 0.5 |
| >1 year | 1 (0.3) | 4 (0.4) | 0 | ||
| Total infant deaths | 36 (10.6) | 158 (15.4) | 0.04 | 37 (7.2) | 0.1 |
| Mean matemal age at delivery in years (SD) | 33.1 (3.7) | 30.5 (4.5) | <0.001 | 33.8 (3.7) | 0.001 |
| No (%) of mothers aged <30 years | 333/1668 (20.0) | 2326/5119 (45.4) | <0.001 | 791/5128 (15.4) | <0.001 |
| Odds ratio (95% confidence interval) of mothers aged <30 years v mothers >30 years | 1 | 3.4 (2.9 to 3.8) | <0.001 | 0.7 (0.6 to 0.8) | <0.001 |
| No (%) of women treated with: | |||||
| In vitro fertilisation | 1262/1673 (75.4) | — | 3456/4605 (75.0) | 0.8 | |
| Intracytoplasmic sperm injection | 411/1673 (24.6) | — | 1149/4605 (25.0) |
*
Differences of means of continuous parametric data were analysed with the use of Student's t test.
†
Distributions between groups were compared by using Pearson's χ2 analyses.
‡
Stillborn children were not included in the cohorts and analyses.
§
The sex was known for 3352 IVF or ICSI twins, 10 215 control twins, and 5130 IVF or ICSI singletons.
¶
Children, who died were registered until 31 December 2000. After that point data from the Danish registry of causes of deaths were not updated. Children who died after delivery were included in the cohorts.