When determining the duration of a pregnancy, healthcare providers use a method called gestational age calculation. This calculation is based on the first day of your last menstrual period (LMP). Despite the fact that conception typically occurs around two weeks after your LMP, healthcare providers consider the start of your pregnancy as the first day of your LMP. This means that even though you may not have conceived until later in December or early January, you are still considered to be four months pregnant based on this calculation method. It's important to remember that these calculations are estimates and can vary depending on individual factors. If you have any concerns or questions about your pregnancy timeline, it's best to consult with your healthcare provider for personalized guidance and clarification.