The term “pregnant” is not biologically accurate for a spider, which is an oviparous species; the correct term is “gravid.” A female spider is considered gravid when she is carrying fertilized eggs internally. Unlike mammals, spiders do not undergo a prolonged gestation period where the offspring develop inside the mother. Instead, the female stores sperm from a single mating and then produces and carries the eggs until she is ready to lay them into a protective silk sac. This internal process leads to distinct physical changes that signal the impending egg-laying phase.
Visual Indicators of a Gravid Spider
The most striking indication of a gravid spider is the dramatic enlargement of the abdomen, known scientifically as the opisthosoma. This body segment, which houses the digestive organs and reproductive system, becomes significantly swollen and often appears spherical or highly distended. The volume of developing eggs causes the abdomen to balloon to a size that can be two to three times its normal proportion.
This size change creates a noticeable visual imbalance, as the cephalothorax, the spider’s fused head and chest section, remains the same size. The abdomen may appear taut and glossy, with the exoskeleton stretched to its limit. In many species, the stretching of the cuticle can reveal the lighter, underlying membrane, sometimes exposing a change in the spider’s overall color or pattern.
A gravid female will often move more slowly and deliberately due to the weight and bulk of her abdomen. She may spend more time stationary, focusing on maintenance of her web or seeking a secure, secluded location for the eventual egg-laying. This disproportionate size and change in mobility are the primary visual cues that a spider is carrying eggs rather than simply well-fed.
Distinguishing Gravidity from Recent Feeding
A challenge in identifying a gravid spider is that a recent, large meal can also cause the abdomen to swell considerably. To differentiate between a feeding-related bulge and gravidity, observe the spider over time. An abdomen distended from a meal will typically reduce in size and firmness within 24 to 48 hours as the spider digests the prey.
In contrast, the swollen abdomen of a gravid spider remains large, firm, and consistently distended until the eggs are laid. The eggs represent a fixed mass that digestion cannot reduce. Behavior also offers a clue; a recently fed spider may still be active in hunting or web-building, while a gravid female focuses on finding a safe nesting site.
Gravid spiders often enter a period of reduced activity, sometimes abstaining from hunting altogether as they conserve energy. This pre-oviposition period is characterized by the internal development of the eggs, which requires significant metabolic resources. The sustained, disproportionate size of the opisthosoma, regardless of food intake, is the definitive sign of a female carrying eggs.
The Egg Sac Stage
Once the female lays her eggs, she is no longer technically gravid, as the eggs are now external and encased in an egg sac. This sac is constructed from specialized silk that is often thicker and more durable than the silk used for webs. The appearance of the sac is highly species-specific, varying in shape, color, and texture.
Many species create a spherical or pear-shaped sac, which can range from creamy white to brown, and may be smooth or covered in debris for camouflage. The sac’s primary function is protection, shielding the developing eggs from desiccation, temperature fluctuations, and predators. A single sac can contain anywhere from a few dozen to over a thousand eggs, depending on the spider’s size and species.
The spider’s post-laying behavior is determined by her species. Wolf spiders attach their sac to their spinnerets and carry it everywhere, while nursery web spiders often carry the sac in their jaws. Cobweb spiders usually suspend the sac within their webs, guarding it until the spiderlings emerge. The presence of the silk sac is the clearest evidence of successful reproduction.
Common Misconceptions and Safety
A common misconception is that a mother guarding an egg sac becomes highly aggressive and will actively seek to attack humans. In reality, a mother spider protecting her eggs is primarily defensive. Her behavior is focused on protecting the sac, and any perceived “attack” is typically a defensive strike against a perceived threat to her offspring.
Finding an egg sac indoors does not automatically mean an overwhelming infestation. The sac must first hatch, which takes several weeks, and the spiderlings that emerge are extremely small. If a sac is discovered in an undesirable location, the safest removal method is to use a container and paper to gently capture the mother and her sac together.
Physical removal should be done cautiously, as disturbing a guarded sac can cause the mother to react defensively. If the spider is not a medically significant species, the most straightforward approach is to safely relocate the entire unit—the mother and her sac—to an outdoor location away from the home. This respects the spider’s maternal instinct while removing the sac before it hatches.