66. In a number of states,71 so-called hidden data will be added to the birth certificate, if the principle of presumption of the child’s descent from the mother’s husband cannot be applied or if paternity has not been recognised or established by a court. This hidden data includes the name of the father indicated by the person reporting the birth, and if there is no such indication chosen by the head of the registry office. However, the mother’s surname at the time of the child’s birth is indicated as the father’s surname.72 67. In some countries,73 certain other non-legal data can be included in the register. For example, the name of the surrogate mother, of the provider of the egg cell or sperm. These data on the biological or genetic filiation are not mentioned in the birth certificate but could be necessary to trace the biological or genetic origin of the child. 68. When the civil registry office registers the birth of a child found or born and abandoned by a mother without identity documents at a personal health care facility, data about the child’s parents are not recorded, and a remark is entered in the birth record: “Child found, parents unknown” or “Child left in personal health care institution, parents unknown.”74 6. Registration in case of absence of proof of the exact place, time, or date of birth 69. A few different types of case, where the exact place, time or date of birth may be uncertain, have to be distinguished: (i) birth in an airplane, ship, or a train; (ii) birth abroad; (iii) other circumstances which make the place, time, or date of birth difficult to establish. i. Birth in a means of transport 70. Special rules exist in several states for births that took place on board of a ship or aircraft. Two states reported that such birth must be reported to the civil registrar at the first port or airport of arrival if this is located in the country.75 71. In another state,76 law stipulates that if a child was born on a ship, a document stamped by the ship’s captain which indicates the time and gender of the child’s birth, mother’s first name, surname, social security number 71. 72. 73. 74. 75. 76. E.g. Poland. Idem. E.g. Switzerland. E.g. Lithuania. Croatia and Denmark. Lithuania. Issues at stake: international standards and stocktaking of national situations ► Page 25