Dog pregnancies average 63 days, so the time between conception and beginning to show symptoms is short. You won't see physical change for the first half of gestation. Before that, you might notice changes to your dog's behavior.
A pregnant dog doesn't typically show physical signs of her condition until about day 40 of pregnancy. About this time, her belly will begin to noticeably expand, continuing to do so for about three more weeks -- her nipples will also enlarge; in the final week of the pregnancy, they may leak milk.
Your dog may show early signs of pregnancy in her behavior. For example, around weeks three and four of gestation, a pregnant dog may experience a few days of morning sickness and vomiting. She may not have much of an appetite or will eat smaller meals throughout the day instead of larger portions. Lethargy and listlessness are typical throughout a pregnancy.
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