Inclusion Criteria:

Potential subjects must:

• Be healthy women, who are sexually active, at risk for pregnancy and desiring contraception.
• Be within the age range of 18 through 40 years inclusive.
• Be at low-risk for human immunodeficiency virus (HIV) or sexually transmitted disease (STD) infection and currently have a single sex partner (minimum 4 months) who is also at low-risk for HIV or STD.
• Have a negative urine pregnancy test prior to enrollment.
• Have normal cyclic menses with a usual length of 24 to 35 days over the last 2 cycles or at least one spontaneous, normal menstrual cycle (2 menses) since delivery, abortion, or after discontinuing hormonal contraception/hormonal therapy.
• Be willing to accept a risk of pregnancy.
• Be willing to engage in at least 4 acts of heterosexual vaginal intercourse per month for a period of 6 months.
• Be willing to be randomized to either study treatment.
• Be willing to only use the study product as the primary method of contraception over the course of the study with the exception of emergency contraception (EC), when indicated.
• Be capable of using the study product properly and agree to observe all study directions and requirements.
• Be willing to keep a diary to record coital information, product use information, information about the use of other vaginal products, and sign and symptom data of subject and partner.
• Agree not to participate in any other clinical trials during the course of the study.
• Be willing to give written informed consent to participate in the trial.

Exclusion Criteria:

Potential subjects must not:

• Have a history of allergy or sensitivity to spermicides or products containing N-9.
• Have had 3 or more urinary tract infections (UTI) in the past year.
• Have UTI by urine culture or symptomatic yeast vaginitis or symptomatic bacterial vaginosis diagnosed by wet mount unless treated and proof of cure is documented.
• Be pregnant, have a suspected pregnancy or desire to become pregnant during the course of the study.
• Have a history of infertility or of conditions that may lead to infertility, without subsequent intrauterine pregnancy.
• Have any contraindications to pregnancy (medical condition) or chronic use of category D or X medications.
• Have had more than one sexual partner in the last 4 months.
• Have shared injection drug needles within the past 12 months.
• Have or have been suspected to have HIV infection.
• Have been diagnosed with genital herpes simplex virus (HSV), with the first occurrence (initial episode) within three months prior to screening.
• Have 3 or more outbreaks of HSV within the last year.
• Have been diagnosed with any other STDs (including trichomonas) in the 6 months prior to the screening visit (with the exception of Human Papilloma Virus [HPV]).
• Be lactating or breastfeeding.
• Have any clinically significant abnormal vaginal bleeding or spotting within the month prior to screening.
• Have any clinically significant abnormal finding on pelvic examination or baseline labs, which in the view of the investigator, precludes her from participating in the trial.
• Have clinically significant signs of vaginal or cervical irritation on pelvic examination.
• Have had vaginal or cervical biopsy or vaginal surgery within 3 months prior to screening.
• Have used vaginal or systemic antibiotics or antifungals within 14 days prior to screening or randomization.
• Have had a Depo-Provera® injection in the 10 months prior to enrollment.
• Have an abnormal Pap smear with high grade squamous intraepithelial lesion (HSIL), atypical glandular cells (AGC) or ASC-H (atypical squamous cells, cannot exclude HSIL) within the last 12 months.
• Have an abnormal Pap smear with low-grade squamous intraepithelial lesion (LSIL) or ASCUS-HPV HR positive unless resolved by colposcopy.
• Have a Cervical Intraepithelial Neoplasia (CIN) diagnosis by biopsy within the last 12 months.
• Have a history or a current diagnosis of cervical cancer.
• Consume (on average) greater than three drinks of an alcoholic beverage per day.
• Have a past history (within twelve months) or current history of drug abuse [recreational, prescription or over-the-counter (OTC)].
• Have taken an investigational drug or used an investigational device within the past 30 days.
• Have previously participated in or completed this study or any other phase III study of C31G.
• Have issues or concerns (in the judgment of the investigator) that may compromise the safety of the subject or confound the reliability of compliance and information acquired in this study.