The best thing is to take your guinea pig to your vet for a pregnancy diagnosis. They can diagnose a pregnancy in various ways depending on how long your guinea pig has been pregnant.
It is important to determine the number of pups in the womb so that you can be sure the sow (female guinea pig) has given birth to all the pups and is not having difficulties delivering them all (which helps to ensure no pups have remained stuck in the womb).
Your veterinarian can advise you on how to care for the pregnant female and the newborn pups.
There are a number of serious conditions that can affect a pregnant sow, including pregnancy toxaemia, severe difficulty giving birth (dystocia), infected mammary glands (mastitis) and uterine prolapse, and high mortality of newborn guinea pups is common.
Your veterinarian can advise you on signs to watch out for and prompt veterinary treatment is vital. If you do notice anything out of the ordinary about your guinea pig (e.g., she is not eating well, she seems depressed or quiet), contact your veterinarian promptly for advice.
There are vets who specialise in exotic animal medicine and surgery, including guinea pigs; your local vet can refer you to a guinea pig vet if necessary.
For more information see this article.
Howkins M, Bishop C (2012) Disease Problems of Guinea Pigs. In: Quesenberry K, Carpenter J (eds) Ferrets, Rabbits, and Rodents, Third. W.B. Saunders, pp 295–310
Kaiser S, Krüger C, Sachser N (2010) The Guinea Pig. In: The UFAW Handbook on the Care and Management of Laboratory and Other Research Animals: Eighth Edition. Wiley-Blackwell, pp 380–398
Quesenberry K (2022) Breeding and Reproduction of Guinea Pigs. In: MSD Veterinary Manual. https://sit2.msdvetmanual.com/all-other-pets/guinea-pigs/breeding-and-reproduction-of-guinea-pigs. Accessed 31 Aug