Table I.

Overview of background characteristics by VTS among ART singleton deliveries born in Norway between July 1984 and December 2013 (n = 20 410).

Characteristics	ART singleton without VTS (n = 17 291) %	ART singleton with VTS (n = 638) %	ART singleton uncertain VTS (n = 2481) %
Maternal age (y)
25–29	21	16	21
30–34	43	40	40
35 and older	37	44	39
Maternal parity
0	65	63	70
1	28	30	25
2 or more	6	7	5
Maternal marital status
Married/cohabitating	98	98	96
Single	2	2	4
Maternal chronic diseases during pregnancya
No	88	87	88
Yes	12	13	12
Year of birth
<1992	2	2	7
1992–1998	10	15	15
1999–2006	31	42	36
2007 and later	57	40	42
Sex
Male	52	49	49
Female	48	50	50
Uncertain	0.1	0.3	0.2

When information from the early ultrasound was missing, the status of vanishing twin syndrome (VTS) was defined as ‘uncertain’.

^aChronic medical conditions included asthma, hypertension, heart disease, kidney disease, rheumatoid arthritis, epilepsy, thyroid disease and diabetes.

The χ² tests comparing the distribution of background characteristics between the groups of ART singletons yielded P-values < 0.001. The only exception was maternal chronic diseases during pregnancy, which showed no difference across the groups of ART singletons (χ²P-value 0.8).