Vitamin D deficiency is common among otherwise healthy pregnant women and may have consequences for them as well as the early development and long-term health of their children. However, the importance of maternal vitamin D status has not been widely studied.

The present study is divided into a societal experiment (1) and a case-cohort study (2):

1. The present study includes an in-depth examination of the influence of exposure to vitamin D early in life and during critical periods of growth for development of type 1 diabetes (T1D), type 2 diabetes, gestational diabetes, pre-eclampsia, obesity, asthma, arthritis, cancer, mental and cognitive disorders, congenital disorders, dental caries and bone fractures during child- and adulthood. The study is based on the fact that mandatory fortification of margarine with vitamin D, which initiated in 1937, was terminated in 1985.

   Apart from determining the influences of exposure prior to conception and during pre- and postnatal life, the investigators examined the importance of vitamin D exposure during specific seasons and trimesters, by comparing disease incidence among individuals born before and after the fortification.

2. Additionally, a validated method was used to determine neonatal vitamin D status using stored dried blood spots (DBS) from individuals who develop the aforementioned disease entities as adults and their time and gender-matched controls.

Unparalleled, the study will help determine the effects of vitamin D exposure during critical periods in life. There are a sufficient number of individuals to verify any effects during different gestation phases and seasons of the year. The results, which will change our current understanding of the significance of vitamin D, will enable new research in related fields, including interventional research designed to assess supplementation needs for different subgroups of pregnant women. Also, other health outcomes can subsequently be studied to generate multiple new interdisciplinary health research opportunities involving vitamin D.

Inclusion Criteria:

1. Born in Denmark around the fortification period
2. Sufficient amount of blood from the DBS

Exclusion Criteria:

1.Born outside Denmark

1. Born outside the fortification period
2. Insufficient amount of blood from the DBS or DBS not found

Entire Danish population born between June1983-May1985 and September 1986-August 1988.

For the part of the study using vitamin D measurements from DBS:

The Danish Civil registration system was used to identify all live-born children during 1981-2002 (n = 1,360,466), and a random subcohort thereof was sampled for 25(OH)D analyses to be used across the D-tect studies (n = 3585)