How to SEX Guinea Pigs ⚥

Getting your guinea pig’s gender right is JOB ONE. Please do not put any guinea pigs together until you know their sex for certain. Don’t believe what anyone tells you —  trust but verify — unless it’s a pet store, then do not trust at all. Literally everyone has made a mistake at one time or another on sexing guinea pigs — pet stores: all the time, but even vets, rescuers, shelters — everyone. We’re all human. While you can usually trust vets, rescues, and shelters, don’t pay that high price for one person’s mistake. Sexing guinea pigs is not that hard when you know what to look for, but younger animals can be trickier so get comfortable with how to confirm gender before adopting. You’ll save yourself from the many risks and financial burden of an unexpected pregnancy.

If you’re not sure, keep them separated until you are.

When looking at various websites, photos, and explanations of how to sex a guinea pig, keep in mind that it’s easier to sex a guinea pig when you have one of each gender for comparison. Since most of us don’t have both sexes to compare, we need to learn the ways of confirming gender by other means, which are listed below.

Also, the oft-touted “Y” or “i” method can be misleading upon a quick visual inspection. A female can look like it has a ‘dot’ there and males can seem to have a “Y” — especially younger guinea pigs — so please go the extra mile and physically confirm your assessment per the information below.

(Click any image below to enlarge or flip through them — in separate sections.)


  • Feel GENTLY just above the genital area. You should be able to feel a ridge indicating a shaft of the penis just under the skin.
  • Press down GENTLY just above the genital area towards the head of the guinea pig. You should be able to get the penis to extrude.


  • Very GENTLY spread the vulva area a bit. If it separates into a “Y-shape,” then it’s a female.
  • Feel GENTLY right above the genital area. There should be NO feeling of a ‘ridge’ or shaft that would indicate a penis.

Correcting Common Internet Myths

  • Waiting to sex babies
    You do not need to wait very long to sex baby guinea pigs. If you had an accidental litter or adopted a pregnant guinea pig, you need to be planning your housing and big picture strategy on keeping or rehoming them. Since their genital area is small, it can be a bit tricky, so if you sex them early, be sure to CONFIRM your assessment again AFTER they are 2-3 weeks old.
  • Male Fertility and WHEN to separate from mother
    Males can be fertile as early as 3 weeks old. However, if your male is small and not showing any signs of chasing or mounting behavior, you can leave him with the mother for another week. Males should be separated at 3-4 weeks of age — no longer than that.

Exploring Common Pitfalls

  • Pregnant guinea pig with her mate?
    Don’t leave a fertile male in the cage with a pregnant female. She could miscarry at any time (although those odds are low), but once she delivers (and you’re rarely around for the blessed event), females go into heat in a big way right after delivery (within an hour) and the male will be only too happy to oblige. Back-to-back pregnancies are bad for the mother.1 If you leave the male in with her, pull him out when she’s getting close. Gestation is about 8.5 to 10.5 weeks (59-72 days, roughly 2 months plus). If you are planning to have the male neutered, doing it while she’s pregnant is good timing.

    Also, if the male IS in the cage when she delivers — as in “OOPS, we didn’t know she was a he” — now you are on a 10-week pregnancy watch and while dealing with one unexpected pregnancy, you won’t know for quite a while whether you have another unplanned pregnancy on the way.
  • “Oh, they’ve been together a long time and he never bothers her.”
    Stop reading this page and go separate your guinea pigs. You are playing with fire.
  • “They get supervised play dates.”
    Oh my, no. Please read below to see the results of turning your head for just a few seconds, or answering the phone, or replying to a text. Supervised playdates between a fertile male and female are an accident waiting to happen, and a costly one at that.

Why sexing correctly is so MONUMENTALLY IMPORTANT

Please do not live in a fantasy world where having one litter is a good thing, or believe that they somehow biologically need to have one litter, or this is how you teach your kids about the birds and bees. The consequences are almost always more than you bargained for, not to mention the negative impact on the bigger picture.

Things to consider about the results of breeding:

  • Litter size
    Guinea pigs have between 3 and 4 pups on average. 7 is not uncommon. They could all be males.
  • Lots of males
    Two males can be paired up and bonded successfully most of the time. Conversely, three or more males together is a challenge most of the time and not recommended. The recommended cage size for 2 males is a 2×5-grid C&C cage (with a loft if possible) — which is 6.5 feet long. An even bigger cage is recommended if you are going to attempt to house 3 males together. Babies and young males can get along well until their hormones start kicking in between 5-12 months of age. That’s when the trouble starts.
  • Female health risks with pregnancies
    Pregnancies can be difficult for very young females or older females. Be sure to have vet care lined up ahead of time in case of emergency.
  • MORE LITTERS if you don’t separate the babies in time
    It’s easy to miss the window on separating the sexes in time. If a fertile male gets to more than one female, things can quickly spiral out of control.
  • Caging requirements for a pregnancy
    Trying to figure out caging requirements BEFORE you know what genders you will have is tricky. Will they be females that can be grouped together? Or males requiring separation into pairs? Will the Dad be alone and need a male friend? Will you have a big divided cage for a while, allowing the babies to run back and forth between the Mom and Dad until the males have to be separated? Will the Mom be alone and need a friend? Will one or more guinea pigs need to be rehomed in a month? How much space do we have in our home to provide proper caging environments? Money for outfitting multiple cages? Time to feed and clean multiple guinea pigs? Someone to take care of multiple guinea pigs when we are gone? There is a long list of questions to be considered.
  • Some not uncommon guinea pig combinations can result in LETHAL WHITES
    If you don’t know what a Lethal White is and what breed combinations result in a 25% chance of a deformed, expensive vet care-required baby with a limited life-span, then don’t breed your pets. This isn’t a site to teach about breeding, so please do the research. If you have an unintentional litter with an all-white baby with red eyes that seems to have either eye or dental or other trouble, then you need to research “Lethal White” guinea pigs.
  • Neutering and spaying keeps getting more expensive
    It seems the pandemic has helped spike vet prices and reduced the availability of vet care in general, especially where elective surgeries are concerned. Guinea pigs need to be seen by a vet that specializes in small or exotic pets. The majority of cat and dog vets won’t even see guinea pigs.
  • Neutering males will not reduce aggressive behavior much
    Neutering males will not make it easier to combine them into larger groups. The only reason to neuter male guinea pigs is to prevent pregnancy so they can be paired with a female.
  • ONE neutered male per female or group of females
    Two or more neutered males will fight over females. So, even if you get males neutered, you can only put one neutered male in with one or more females. However, ONE neutered male with one or more females is a great combination.
  • Rehoming
    The pandemic has also triggered a huge demand for guinea pigs since around April 2020 (more people staying home and wanting pets). It hasn’t let up much yet (as of spring 2021). So, for now, your odds of getting your guinea pigs adopted out are HIGH because demand is still high. That being said, it remains to be seen what happens in the coming months and years as life gets back to ‘normal.’ Those of us in the rescue world are very concerned about the swing of the pendulum on the supply and demand side of pets. It may become more difficult to find a good home as more guinea pigs end up being rehomed — increasing supply and reducing demand. We can only wait and see.
  • $$$$
    It can easily cost you many hundreds of dollars in unexpected costs if you don’t correctly sex your guinea pigs AND keep fertile males separated from fertile females. Hopefully, all the other reasons are compelling enough, but costs can add up quickly as well.

Additional Resources

  • Sexing Guinea Pigs (Guinea Pig Gender Determination)
    A veterinary-focused website with no other agendas
    Provides a thorough explanation with more images, although their comments about not handling babies are overwrought — the sooner you know what you have or are looking to adopt or rehome, the better, and mothers are fine with you handling babies. But, the site includes good suggestions on being cautious when examining new guinea pigs that are not in your home — especially pertinent for adopting new guinea pigs.
  • What Sex is my Guinea Pig?
    An Aussie veterinarian site with the basics.

1 Back-to-Back Pregnancies, especially when it’s a young mother to start with:
Young guinea pigs AND pregnant mothers both have an increased demand for calcium and other nutrients — either in their young, growing years OR if they have babies inside needing extra calcium for their growing bones. When it’s a young and pregnant female, it’s extra taxing on her overall nutritional requirements. Then, if after delivering babies, she gets pregnant again right away, be mindful that she is trying to nurse multiple babies, provide them with good nutrition, and she’s needing good nutrition herself to help recover from pregnancy. That can stunt the growth of young females and they can end up being a smaller, less robust version of their original potential.