If you travel to Pakistan in the summer months be prepared for the very hot temperatures (40 degrees plus) and mosquitoes carrying malaria and dengue organisms. Although most cars, buildings and apartments have AC, there are power outages and “load shedding” so unless you have a well serviced back up system/generator you are in for some roasting.
Pakistan is renowned for strikes and this may affect the courts and any public transportation. Consider court strikes and other institutional shut downs when making plans.
Politics and terrorism
If there is any political unrest in the city then curfews can be placed which may impede on your movement around the city. These have been rare but if you are going at a time of a general election this may become frequent and dangerous
Cost of living
This will depend on whether you are staying with relatives, have an apartment or staying in a hotel. Overall consider your costs when staying in Karachi as hotels and apartments are not cheap and have very similar pricing to the UK.
Work related issues
If your partner is taking annual leave for the waiting and orientation period, keep in mind you maybe waiting up to 8 weeks for a baby. Overall the timeframe for the total stay in Pakistan is approximately between 4-6 months, before you can bring baby home to the UK.