For maxmum leaving Canada is 6month. If you want staying in Japan over the 6month with very specific emergency case, you can apply RETURNING PERMIT at Immigration of Canada. If they accept that your case is emergency , they will issue the letter of returning permit to you. But this permit is maximum 1year and 6month. If you stay in Japan over the period of returning permit , your landed immigrant status will automatically erase. And also , you case of staying in Japan with over 6 month is seems not on the field of emergency situation. You can birth your child in Canada. They can only accept this matter for very special situation. May be you can not get the permission for your above perposes.