Women who are overweight (BMI, body mass index over 30) are more likely to have difficulty conceiving and are also more likely to develop complications during pregnancy. Complications such as miscarriage, high blood pressure and diabetes are all more likely, as are difficulties during labour and delivery. If you are undergoing fertility treatment, this is less likely to be successful if you are overweight. It is therefore recommended that you maintain a healthy weight and pre-pregnancy you can use the BMI healthy weight calculator to work out what your ideal weight should be. If you are already pregnant, you should discuss this with your midwife or doctor.
You can get further advice from https://www.nhs.uk/live-well/healthy-weight/.