This study will evaluate the levels of hydroxyprogesterone caproate and it's metabolites in blood throughout pregnancy

There will be 3 cohorts of subjects

Cohort 1 (6 subjects) will have daily blood draws in the morning at approximately the same time each day for one week following their first dose of Makena and will have blood drawn immediately prior to 2 successive Makena doses during Epoch 1 (24 - 28 weeks) and Epoch 2 (32 - 36 weeks)

Cohort 2 (8 subjects) will have blood drawn to determine the trough concentration 7 days after their first dose of Makena. Subjects will have daily blood draws for one week following a dose of Makena given in Epoch 1 (24 - 28 weeks) and blood drawn immediately prior to 2 successive doses during Epoch 2 (32 - 36 weeks)

Cohort 3 (16 subjects) will have blood drawn to determine the trough concentration 7 days after their first dose of Makena. Subjects will have blood drawn immediately prior to 2 successive doses in Epoch 1 (24 - 28 weeks) and daily blood draws for one week following a dose of Makena given during Epoch 2 (32 - 36 weeks)

A maximum of 10 subjects will be monitored on selected days following a completed course of Makena therapy to determine the terminal elimination phase.

Inclusion Criteria:

• Age ≥ 18 years.
• Singleton gestation.
• Gestational age 16 weeks 0 days to 20 weeks 6 days.
• Previous singleton spontaneous preterm delivery

Exclusion Criteria:

• Multifetal gestation.
• Known major fetal anomaly or fetal demise.
• Progestin treatment in any form in the 4 weeks prior to study entry.
• Heparin therapy during current pregnancy or history of thromboembolic disease.
• Maternal medical/obstetrical complications including hypertension requiring medication or seizure disorder.
• uterine anomaly other than fibroids
• Known hypersensitivity to hydroxyprogesterone caproate injection or its components.
• Any significant medical disorder that, in the opinion of the investigator, would be a contraindication to the use of the drug or would preclude accurate evaluation of the subject's condition or outcome or compromise the subject's safety in the study.