Primary purpose of the study is to assess the concentration of bimekizumab in mature human breast milk.

Inclusion Criteria:

• Study participant must be at least 18 years of age at the time of signing the informed consent.
• Study participant is being treated with commercial bimekizumab per locally approved prescribing information and in accordance with her treating physician.
• Study participant is breastfeeding and intends to breastfeed throughout the Sampling Period.
• The decision to treat with bimekizumab and to breastfeed is made independently from and prior to the study participant consenting to participate in the study.
• Study participant must be on bimekizumab treatment for at least 12 weeks after delivery and since starting/restarting/continuing bimekizumab prior to sampling (prior to start of the Sampling Period).
• A female study participant is eligible to participate if she is not pregnant and does not plan to become pregnant during the study.

Exclusion Criteria:

• Study participant has any medical, obstetrical, or psychiatric condition that, in the opinion of the Investigator, could jeopardize or would compromise the study participant's ability to participate in this study.
• The infant has any abnormality noted on physical examination or medical history that, in the opinion of the Investigator, may jeopardize or compromise study participation.
• Study participant has a history of chronic alcohol or drug abuse within the previous last year.
• Study participant has history of breast implants, breast augmentation, or breast reduction surgery.
• Study participant plans a surgical intervention during the Screening Period and Sampling Period or anticipates having surgery while participating in the Sampling Period (does not apply to tubal ligation).
• Study participant or her infant has participated in another study of an investigational medicinal product (IMP) (and/or an investigational device) within the previous 6 months or is currently participating in another study of an IMP (and/or an investigational device), unless the study is UCB PS0036 or a bimekizumab registry study.
• Study participant or her infant has previously participated (ie, entered the sampling period) in this study.

Due to the small sample size in this trial, individual patient-level data cannot be adequately anonymized as there is a reasonable likelihood that individual participants could be re-identified. For this reason, data from this trial cannot be shared.