Niṣeka and Ādhāna are separate events in the creation of a baby. Niṣeka (निषेक) literally means sprinkling, infusion, dripping and refers to the seminal infusion which leads to impregnation and the vedic ceremony connected with it. Ādhāna (आधान) is better understood from the word garbhādhāna where garbha means womb. It means depositing, placing or impregnating and refers to the place in which anything is deposited or rests, which in the context of birth is the womb. Impregnation or fertilization of the ovum happens after coitus (niṣeka) and can be as far apart as seven days. Many astrologers have come up with various innovative schemes and calculations for knowing this. Yet the method of the great master Parāśara remains the best and most accurate. If you ponder at the philosophy behind those calculations, there would be a lot to learn.