This is a very good example of how difficult can be the biology sometimes.
Different social strata.
Parents working in different jobs, maybe more prone to go in contact with dangerous substances or living in more contaminated areas.
Different diets, maybe relying more in very cheap food choices.
Different country, with different resources and laws that could lead this women to not be fully aware of the local opportunities available in their pregnancy, or how to ask for their rights.
Recent campaigns of hate against immigrants in Swedden, that could lead to a worse treatment in hospitals by one or a few racist members of the staff.
Not able to pay for a premium healthcare while giving birth.
And immigrants live in countries with poor environmental laws and endure a dangerous travel that can damage their health. Dangerous levels of heavy metals were found in workers that had to burn hardware and recycle computers in a third world country to pay for their travel to Europe, for example. Somali people also eat a lot of migrant fish (aka: mercury with fins).
Vitamin D is just one in a thousand of possible explanations